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Speedwork for Ultrarunners: Why Even Slow Runners Need Fast Days

Here is the thing nobody tells you when you sign up for your first ultra: even if your race is twelve hours of power-hiking and slogging, you still need a little speed in your week. Not to win a sprint. To raise your ceiling, because your all-day pace is just a fraction of that ceiling, and when the ceiling goes up your easy pace gets cheaper. I am talking about a small, smart dose: strides, hill repeats, a VO2max session, some threshold running. One quality day a week, maybe two if you are experienced, and you keep everything else easy. This guide is the why and the how, with real reps, real effort zones, and the cautions that keep speed from blowing up your training.

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Why even slow ultra runners need fast days

The argument against speed work sounds reasonable. My race is slow, so why train fast? But it misses how this actually works. Speed work improves all of your running, no matter the pace you race at, because the adaptations it drives sit underneath everything you do.

You are raising the ceiling, not chasing a sprint

Three things get better when you add a little intensity. Your VO2max (how much oxygen your engine can use), your lactate threshold (the fastest pace you can hold for a long time before things fall apart), and your running economy (how little energy it costs you to run at a given pace). All three lift your top end. And here is the part that matters for an ultra runner: your race pace is a fraction of that top end. Raise the ceiling and your all-day effort becomes a SMALLER slice of your max. Same trail, less cost.

Running economy is the sneaky one. When you get more efficient, your slow running gets more efficient too, automatically. So the strides and intervals you do at faster speeds quietly make your shuffle at hour 18 cheaper. There is also good evidence this is not just theory for ultra: in long trail races, VO2max and velocity at VO2max (the speed you hit your VO2max at) show up as real predictors of performance. That is exactly the stuff fast days build.

The four workouts that matter

You do not need a track program. Four kinds of session cover almost everything an ultra runner needs, and for trail runners the hill repeats are the gem because they build strength and speed at the same time while sparing your joints. Here is the menu, with what each one is for and a concrete way to run it.

Workout	What it builds	Effort	How to run it
Strides	Running economy + neuromuscular pop	About 90% of max speed, relaxed, not a sprint	4 to 6 reps of 15 to 20 seconds (roughly 50 to 100 m), full walk-back recovery, tacked onto the end of an easy run a couple of times a week. Quick light feet, tall posture. The lowest-cost, lowest-risk speed there is.
Hill repeats	Power, strength, VO2max with less pounding	RPE 8 to 9 out of 10, hard but controlled	A classic ultra staple is 3 minutes hard uphill, 3 minutes easy back down, 4 to 6 times. Pick a grade you can still RUN, not one that forces a walk, and skip the technical stuff so you can push the effort.
VO2max intervals	Raise the ceiling (VO2max + velocity at VO2max)	90 to 95% of max heart rate, ~5K race effort	Work bouts of 2 to 4 minutes with roughly 1:1 easy-jog recovery, 4 to 8 reps (think 5 to 6 x 3 min, or 5 x 1000 m). Start with 2 to 3 reps if you are new to it. Run them somewhere smooth so you can hold the effort.
Threshold / tempo	Lift lactate threshold (your sustainable speed)	85 to 90% of max HR, comfortably hard	15 to 30 minutes of comfortably-hard running, either steady or in cruise intervals (like 3 x 8 min with 2 min jog). A common target is 30 to 60 minutes of threshold work per week. The single most ultra-transferable session here.

Always bookend the hard sessions with a real warm up (15 to 20 minutes easy plus a few strides) and an easy cool down. The work bouts are the point, but jumping straight into 5K effort cold is how you tweak something. If you only ever do ONE of these, make it the threshold work, it carries over to ultra pace the most directly.

Run them by effort, not by some pace off the internet

If you are a slower ultra runner, do not go looking up interval paces from a road 5K plan and try to match them. Run by feel and by heart rate. The effort is what drives the adaptation, not the number on the watch, and your number is going to look slow next to a road runner. That is fine. It changes nothing.

The three efforts, in plain language

VO2max intervals live around 90 to 95% of your max heart rate, which feels like about your 5K race effort: hard, breathing heavy, repeatable for a few minutes but not a place you could camp out. Threshold and tempo runs are gentler, roughly 85 to 90% of max HR, comfortably hard, the pace where you can only squeeze out a few words at a time. Strides are about 90% of your top-end speed but relaxed and smooth, quick light feet, never a grinding all-out sprint.

Hill efforts are easiest to gauge by RPE, somewhere around 8 to 9 out of 10. The grade does a lot of the work of holding your effort high, which is exactly why hills are so good: you get the intensity without having to run flat-out fast, and the impact on your legs is lower. To turn any of this into honest target paces for YOUR climbs and descents, the grade-adjusted pace tool below does the math for you.

Useful here: the training pace calculator gives you easy, tempo, and interval pace bands off a recent result, and the grade-adjusted pace calculator translates those efforts onto real climbs and descents.

How much, and how often

This is where people get it wrong in both directions. Some skip speed entirely, some turn every run into a hammerfest. The dose for an ultra runner is small and deliberate.

One quality day a week, two at most

For most ultra runners, one hard session a week is genuinely enough, and two is the upper limit for experienced runners with the recovery to back it. Layer strides on top of an easy day or two, because strides are cheap and barely register as fatigue. Everything else in the week stays easy aerobic running and the long run, and that easy volume is what actually carries you through an ultra. The single most repeated coaching line on this is simple: do not sacrifice your volume for intensity.

If you are newer to fast work, start even smaller: a handful of strides and 2 to 3 short intervals or hill reps, and build the volume of the hard part over a few weeks before you reach for the full session. The point is consistency you can recover from, not a hero workout that leaves you trashed for your long run. A blown long run costs you far more than a missed interval ever will.

When to do speed work in your build

Timing matters as much as dose. The mistake is grinding track-style VO2max intervals three weeks out from a mountain 100. By then your training should look like the race, not like a 5K block.

Build the engine early, then make it specific

A clean way to think about it: do most of your faster VO2max and interval work in the base and early-build phases, often a block of 3 to 4 weeks or a couple of mesocycles, while race day is still a ways off. That is when raising the ceiling pays off the most and when you have room to absorb the harder sessions. Get the top end up first.

As the race gets close, shift that intensity toward specificity. Trade the short fast intervals for long climbs, long sustained efforts, big vert, time on the actual terrain, and threshold work at efforts you will really use on race day. Speed does not disappear from the plan, it just changes shape into something that looks like your event. Hill repeats and tempo are the bridge: they keep a little intensity in while already pointing at the demands of a trail ultra.

For where speed fits in the bigger picture, see base building for ultrarunning and how to train for elevation gain and vert.

What a week can look like

Here is one way to slot speed into a normal ultra week without crowding out the long run or your easy mileage. If you only do one quality session, keep the speed day and make the other a plain easy day. The long run is always the priority.

Day	Session
Mon	Rest or easy recovery jog
Tue	Speed day: hill repeats or VO2max intervals (warm up + cool down)
Wed	Easy aerobic + 4 to 6 strides at the end
Thu	Threshold / tempo (or a second easy day if you only do one speed session)
Fri	Easy or rest
Sat	Long run (this is the priority, keep it easy)
Sun	Easy or recovery / back-to-back long

Notice the spacing: the hard day and the long run are far apart, and there is easy running in between so you show up to each one recovered. This is a template, not a law. Move the days to fit your life, just keep the hard stuff and the long run from landing back to back.

The cautions that keep speed from hurting you

Speed is the highest-risk training you do per mile, so a few rules keep it on the right side of the line between helpful and a setback.

Smooth ground, fresh legs, and ease into it

Do your hard efforts on smooth, non-technical ground. Rocks and roots pull your attention off the effort and onto your foot placement, you cannot hold the intensity, and the injury risk on a fast technical descent is real. A buffed singletrack, a dirt road, a track, even a treadmill set on an incline for hill work, all better than a chunky technical trail when you are running fast. And avoid hammering long technical downhills at speed, that is a great way to trash your quads or roll an ankle.

Ease in. Start conservatively, add a rep or a minute at a time over weeks, and never bolt a big jump in intensity onto a week where your mileage also spiked. Stacking a new hard session on top of a sudden volume increase is how overuse injuries happen. If something sharp shows up, especially focal pain over bone, you back off and get it checked rather than pushing through. Speed work is supposed to leave you fitter, not on the couch.

To keep your legs durable enough to handle the fast days, pair this with strength training and injury prevention for ultra runners.

⏵ Stop guessing where speed fits

A generic one-or-two-sessions rule is a fine start, but it does not know your fitness or your race. Summit Line builds a plan that periodizes your speed work, front-loads the intensity early, then converts it to race-specific climbs and threshold as race day nears, all dialed to YOUR actual fitness. Its load-aware Build Watch flags when a hard week and a mileage spike are about to collide, which is exactly when speed work turns into an injury.

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Speedwork for ultrarunners FAQ

Do ultra runners actually need speed work?

Yes, even if your race is mostly hiking and slow grinding. Speed work raises your ceiling, and ultra pace is a fraction of that ceiling, so when the ceiling comes up your easy and all-day paces get cheaper. The gains from VO2max work, threshold work, and strides transfer downward: better running economy makes your slow running more efficient too. You will never out-sprint anybody in a 100 miler, but you will hold your goal pace at a lower heart rate, climb stronger, and have more left in the back half. That is the whole point.

How much speed work should an ultra runner do?

Not much, and that is the trick. One quality session a week is plenty for most ultra runners, and two is the ceiling for experienced ones, with strides on top because strides are easy and barely cost you anything. The rest of your week stays easy aerobic volume and the long run, which is what actually carries you through an ultra. The honest rule coaches repeat is do not sacrifice your volume for intensity. Speed is the seasoning, not the meal.

When in my training should I do speed work?

Earlier rather than later. The common approach is to do most of your faster VO2max and interval work in the base and early-build phases, often a block of 3 to 4 weeks or a couple of mesocycles, while you still have time before the race. As race day gets closer you shift that intensity toward race-specific work: long climbs, long efforts, time on terrain, and threshold work at efforts you will actually use. You do not want to be chasing track-pace intervals three weeks out from a mountain 100. Build the engine early, then make it specific.

What kind of speed workouts are best for trail and ultra runners?

Four cover almost everything. Strides (4 to 6 x 15 to 20 seconds) for economy and neuromuscular pop, basically free. Hill repeats (something like 4 to 6 x 3 minutes hard uphill) for power and VO2max with way less pounding than flat speed. VO2max intervals (2 to 4 minute bouts at 90 to 95% of max heart rate, roughly 1:1 recovery) to raise the ceiling. And threshold or tempo running (15 to 30 minutes comfortably hard) to lift your sustainable pace, which is the most ultra-transferable of the bunch. Hills are the gem for trail runners because they build strength and speed at once and protect your joints while doing it.

How fast should I run intervals if I am a slow ultra runner?

By effort, not by some pace off the internet. VO2max intervals should sit around 90 to 95% of your max heart rate, which feels like roughly your 5K race effort, hard but repeatable for a few minutes at a time. Threshold runs are gentler, around 85 to 90% of max HR, comfortably hard, the pace where you could only get a few words out at a time. Strides are about 90% of your top-end speed but relaxed, not an all-out sprint. Your actual paces will look slow compared to a road 5K runner, and that does not matter at all. The adaptation comes from the effort and the time you spend at it, not the number on the watch.

Will speed work make me faster in a 100 miler or just a 5K?

Both, but through the back door. It will not turn you into a fast finisher in the road sense, but VO2max and velocity at VO2max are real predictors of long trail-race performance, so lifting them helps your ultra directly. The bigger win is efficiency. When your top end comes up, your all-day pace becomes a smaller slice of your max, so you cover the same ground for less effort and you fade less at the end. Stronger legs from hill work also climb better and brake better on descents. So no, you are not doing speed work to win a sprint, you are doing it so hour 18 feels less like the wheels coming off.

This guide is for training and educational purposes and reflects current exercise-physiology consensus and reputable ultra coaching practice. It is not medical advice. Speed work is individual: the right dose and effort depend on your fitness, your history, and your recovery. Build up gradually, run by effort, and if you have a medical condition or persistent pain, especially focal pain over bone, see a sports physiotherapist or physician before pushing intensity.