⏵ Course guide · Free

Ruth Anderson Memorial Endurance Run Course Guide

The Ruth Anderson Run is a flat, fast, looped ultra around Lake Merced in San Francisco, with 100K, 50 mile, and 50K options. No mountains, no altitude, no technical trail. Just a certified lake loop, cool coastal weather, and aid every lap. I will walk you through the course, then give you the even-pacing and fueling stuff that makes a fast, repetitive road ultra go right, plus free tools to dial in your numbers.

⏵ Quick facts

The Ruth Anderson Run at a glance

Date: Sat, October 10, 2026
Location: Lake Merced, San Francisco, CA (start at the foot of Sunset Blvd)
Distances: 100K, 50 mile, and 50K
Course: Certified, mostly flat 4.5 mile (about 4.48 mi) loop around the lake
Surface: Paved path with a groomed dirt shoulder for roughly 90% of the loop
Elevation gain: About 100 ft of gain and loss per loop, low total vert
Cutoffs: Generous overall; 50M has a hard 44.2 mi checkpoint at 11 hrs (15 min/mi)
Series: PAUSATF / USATF Mountain Ultra Trail (MUT) event, not a WS or UTMB qualifier

Note: the next edition is Saturday, October 10, 2026. The course runs a certified, mostly flat 4.5 mile (about 4.48 mile) loop around Lake Merced, repeated to make each distance. Always check the date, exact start time, lap counts, and the current per-distance cutoffs on the official race site before you plan your race.

The course

The Ruth Anderson Run is a looped road ultra around Lake Merced, a freshwater lake on the southwest side of San Francisco. The certified loop is mostly flat at about 4.5 miles (roughly 4.48 miles), and you repeat it to build the 50K, 50 mile, or 100K distance. It is a paved path with a groomed dirt shoulder for roughly 90% of the loop, and only about 100 feet of gain and loss per lap.

A flat, certified loop you will get to know well

Race headquarters sits in the parking lot at the north end of the lake, at the foot of Sunset Boulevard, and every lap brings you right back past it. You run the same loop again and again. The 50K is six laps, the 50 mile is eleven laps plus a short out-and-back, and the 100K stacks on even more. There are no climbs or descents worth naming, and that is exactly what makes this a fast course and a favorite for chasing a personal best at the distance.

The lake loop is paved and certified, so your watch and the mile markers line up cleanly and your splits are honest. That predictability is the whole point. You always know exactly where you are, how far is left, and when you will next see aid, which lets you run a much more controlled race than a point-to-point mountain course ever will.

Aid every lap, and your drop bag every lap

The biggest tactical edge here is the loop itself. There is a fully stocked aid station with water, electrolytes, and a wide spread of food once per loop, plus a second water and drop point roughly halfway around, so you are never more than a couple of miles from support. You can stage your own bottles, gels, and gear at headquarters and hit them on every single lap.

So you can carry almost nothing, which keeps you light and fast, and you can fuel on a precise per-lap schedule instead of guessing across long unsupported gaps. Use the laps as a built-in checklist. A sip, a gel, a salt tab, a quick gear check, the same rotation each time you come through.

The real challenge: flat pounding and repetition

Do not mistake flat for easy. With no downhills to unload your quads and no climbs to switch up the muscles you are using, your stride barely changes for hours, and the repeated impact on a hard, level surface is its own kind of fatigue. The climbs are not what get you here, the flatness is. Late in a 50 mile or 100K, the flat ground that felt fast early turns into a grind on the same tissues, so leg durability on hard ground matters more than people expect.

The other half is mental. Running the same lake loop a bunch of times tests your patience as much as your legs. The runners who do well break the race into chunks, run by feel and by their fueling schedule instead of staring at the lap count, and treat each loop as a small reset instead of counting down some huge total.

Cutoffs and weather

The cutoffs are generous and the race has a name for being forgiving. The 50K basically has no hard cutoff as long as you keep moving. The real hard cutoff is for 50 mile runners, who have to reach the 44.2 mile checkpoint, the 10th lap, by 11 hours into the race, about a 15 minute per mile pace, with a 50K split cutoff earlier in the day. Check the current chart for your exact distance on the official site.

October in San Francisco is usually cool and mild, often overcast near the coast, which is part of why this runs fast. It can get breezy by the lake too, and conditions can swing, so check the forecast and plan a layer you can stash at headquarters and grab on a lap if you need it.

Pacing strategy for the Ruth Anderson Run

A flat, certified, looped course rewards even, disciplined pacing more than almost any ultra out there. There is no terrain to hide a bad split behind. The race is won by out-patiencing yourself.

Pick one steady pace and bank nothing early

On a flat loop the temptation is to bank time while you feel fresh, and that is the classic way to blow up at the Ruth Anderson Run. Nothing about the terrain forces you to slow down, so the discipline has to come from you. Settle into a goal pace you can actually hold for the full distance, run the first few laps a little easier than feels right, and let the even effort carry you.

Use our free race time calculator to set a realistic finish target for your distance, then turn it into a per-lap split you can repeat. Once you know your target lap time, the loop becomes a steady metronome instead of a guessing game.

Run by effort, then by your splits

Even on flat ground, early adrenaline makes goal pace feel way too easy. Anchor the first third of the race to effort, keep it conversational and relaxed, and only let the clock take over once you have settled in. If you have a recent road or trail result, our race equivalent calculator helps you sanity-check whether your goal time is honest before you commit to it on race morning.

If you are coming off hilly training, our grade-adjusted pace calculator is handy in reverse here. It shows how your flat-equivalent pace stacks up against your hilly efforts, so you can set a Lake Merced target that reflects your real flat-ground speed instead of your trail pace.

Plan for the late-race grind

The back half of a flat ultra is a mental and muscular endurance test, not a terrain test. The people who finish strong are usually the ones who held back early and still have a smooth stride at lap eight, nine, or ten. Plan a few small resets in the middle, a caffeine point, a music or playlist switch, a person to look for each lap, so the repetition works for you instead of against you.

And because aid and your drop bag come around every lap, you can make tiny fixes constantly instead of letting problems pile up. A few seconds at headquarters to deal with a hotspot, top off fluid, or take in some real food is almost always worth it on a course this forgiving.

Fueling strategy for the Ruth Anderson Run

A steady, fairly fast pace in cool weather makes fueling simpler than a hot mountain race. But the faster you go, the more carbohydrate you burn. The loop lets you run a precise plan, so build one and follow it lap by lap.

Carbs: keep the engine fed for a fast pace

Sustained running burns carbohydrate quickly, so aim for roughly 60 to 90 grams of carbohydrate per hour, leaning toward the high end if your gut is trained. Use a glucose-plus-fructose blend so you can absorb more than a single sugar allows, and rehearse your exact hourly number on long training runs so it is routine by race day. The cool weather actually helps your stomach handle fuel, so there is rarely a heat excuse to under-eat here.

The loop is your friend. Stage gels and easy-to-grab calories at headquarters and take something on a set schedule every lap or two. Our free ultra fueling calculator turns your weight, goal time, and the mild conditions into a carb, sodium, and fluid number per hour you can rehearse and then go execute.

Sodium and fluid: lighter losses, still a plan

San Francisco coastal weather in October is usually cool and often overcast, so sweat and sodium losses tend to run lower than at a hot race. A moderate sodium concentration, often around 400 to 600 mg per liter of fluid, is a sensible place to start, and you can carry very little because you refill every lap. Drink to thirst, keep electrolytes steady, and do not over-drink just because aid is always right there.

Dial in a plan with our free ultra fueling calculator. Enter your weight, goal time, and the expected mild conditions, and it gives you carb, sodium, fluid, and caffeine targets per hour built for this duration and climate. Then go test the plan in training so race day is rehearsed, not made up on the spot.

⏵ Free calculators for this race

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Ruth Anderson Memorial Endurance Run FAQ

How hard is the Ruth Anderson Memorial Endurance Run?

On terrain alone this is one of the friendlier ultras you can pick. The course is a certified, mostly flat 4.5 mile loop around Lake Merced in San Francisco, only about 100 feet of gain and loss per lap, and a smooth paved path with a groomed dirt shoulder for most of the loop. No altitude, no technical footing, and the cool coastal weather is usually mild. What makes it hard is a different thing than a mountain 100. It is the flat-road pounding with no downhill to give your legs a break, the same loop over and over, and the discipline of holding one pace for 50K, 50 miles, or 100K. Most people run it as a fast PR course, not a survival day.

How much climbing is in the Ruth Anderson Run?

Very little. The certified loop carries only about 100 feet of gain and loss per lap. Over a 50K that adds up to a few hundred feet of total climbing, and even the full 100K stays modest next to a mountain ultra. There are no real climbs or descents to break up your stride, and that is exactly why the course is fast. It is also why the flat pounding on a hard surface, and not the vert, is the thing that wears you down here.

How should I fuel for the Ruth Anderson Run?

The pace here is steady and pretty quick for an ultra, so fuel it like a road race that happens to last all day. Most runners aim for 60 to 90 grams of carbohydrate per hour, leaning to the high end if your gut is trained, because faster sustained running burns carbohydrate quickly. The cool coastal climate keeps sweat and sodium losses lower than a hot race, so a moderate sodium concentration, often around 400 to 600 mg per liter of fluid, is a reasonable place to start. And the loop is a gift. You pass the aid station and your own drop bag every lap, so you can carry almost nothing and refuel right on schedule. Use our free ultra fueling calculator to set your own carb, sodium, and fluid numbers per hour.

What are the cutoffs for the Ruth Anderson Run?

The cutoffs are generous and the race has a name for being forgiving. The 50K basically has no hard cutoff as long as you keep moving. The real hard cutoff is for 50 mile runners, who have to reach the 44.2 mile checkpoint (the 10th lap) by 11 hours into the race, which works out to about a 15 minute per mile pace, with a 50K split cutoff earlier in the day. Always check the exact start time and the current cutoff chart for your distance on the official race site, because those per-distance times can change year to year.

Is the Ruth Anderson Run a good first 100K or 50 miler?

Yes, it is one of the better picks for a first 100K or 50 mile finish. The flat certified loop, the lap-by-lap access to aid and drop bags, the generous cutoffs, and the mild San Francisco coastal weather take away most of the things that wreck first-time ultra runners on a technical mountain course. The tradeoff is mental. You will run the same lake loop a lot of times, so show up with a plan to break the race into segments and stay in it through the repetition.

Does the Ruth Anderson Run qualify for Western States or UTMB?

It is a PAUSATF event in the USATF Mountain Ultra Trail (MUT) series, so it counts toward USATF age-group and competitive standings, but it is not advertised as a Western States or UTMB qualifier. If a specific qualifier is your goal, go check the current qualifying-race lists for that event directly, because qualifier status is set by those races and changes year to year. Most people run Ruth Anderson for a fast time, a solid finish, or a USATF result, not a lottery ticket.

This guide is for planning and training, and it reflects publicly available information about the Ruth Anderson Memorial Endurance Run. Race details, including the date, start time, lap counts, aid stations, and cutoffs, can change year to year. Always check the current specifics on the official race website before you train or travel.