⏵ Course guide · Harriman State Park, Idaho
The Harriman 25K/50K Trail Run loops pine-needle-covered singletrack through Harriman State Park near Island Park, Idaho, about 30 minutes south of West Yellowstone, with Teton views along the way. I will walk you through the course and cutoffs first, then give you a pacing and fueling plan for the 50K, 25K, or 10K. There are free calculators along the way to dial in your own numbers.
These facts come from UltraSignup's official 2026 event listing. The race's own website still shows last year's copy, so double check current details there before you commit.
All distances start and finish near the Parkview Pavilion in Harriman State Park, and the loop structure is simple: the 25K covers the full course once, the 50K covers it twice.
The 25K is a single lap of pine-needle-covered singletrack with views of the Teton Mountains along the way. The 50K simply repeats that same loop a second time, so if you know how the first lap felt, you have a realistic read on what the second one asks of you.
The 2026 lineup replaced last year's 8.1 mile option with a 10K, alongside a new 1K, both starting later than the longer distances and finishing on the same trail system. If you are newer to trail running or bringing family along, these are a lower-commitment way to experience the same course.
There is a single on-course aid station plus support at the start and finish, stocked with cookies, chips, sodas, water, and sports drink. How many times you see it depends on your distance: once on the 10K, twice on the 25K, four times on the 50K. The race runs cupless, so bring your own cup or hydration system.
Neither official source publishes an elevation gain figure, so plan around the course's loop structure and the informal 4:00 PM finish guidance rather than a specific vert number.
Because the 50K is simply the 25K loop run twice, your first lap is the best pacing data you will get all day. Run it at a pace you know you can repeat, not the fastest pace you can survive, so the second loop is a rerun of the first rather than a survival slog.
⏵ Free tools to plan your effort
With one aid station stocked mostly with snacks and drink rather than a full meal spread, plan to carry most of your own calories between visits.
Aim for roughly 60 to 90 grams of carbohydrate per hour and sodium in the 300 to 700 mg per liter range, adjusted upward if mid-August at Island Park runs warm. The single aid station offers cookies, chips, sodas, water, and sports drink, useful supplements but not a substitute for your own carried nutrition on a 50K that visits it only four times.
⏵ Build your fueling plan
Get a carb, sodium, fluid, and caffeine plan per hour built for your weight, your goal time, and a mid-August Idaho day with the free ultra fueling calculator. Browse the rest of the free running tools at the tools hub.
⏵ Train for it with Summit Line
Get a training plan built around YOUR fitness and the kind of repeatable, loop-friendly endurance this course rewards. Summit Line reads your real training, builds the durability your distance asks for, and rehearses your fueling so race day is something you execute, not guess at.
Neither the official race site nor UltraSignup publishes a total elevation gain figure, but the course itself is described as pine-needle-covered singletrack, not a technical mountain course. The 25K is a single loop and the 50K repeats it twice, so the physical demand comes more from time on your feet and heat management on an August afternoon than from a brutal climb. This is a reasonable place to test yourself at either distance without the exposure or rock scrambling you get at a bigger mountain race.
The event lists a soft, informal cutoff rather than a hard disqualifying line: "Completing your chosen distance by 4pm is appreciated." For the 50K starting at 7:00 AM, that gives you 9 hours to cover two loops, generous for a runner who trains for the distance. Still, confirm the current-year cutoff policy on the official race page before you build a tight pacing plan around it.
It has real things going for a first-timer: pine-singletrack terrain rather than technical scrambling, a soft rather than strict cutoff, and a well-supported aid station you pass through four times on the 50K. The lack of a published elevation figure means you should not assume it is flat, but nothing in the course description suggests a brutal climbing day either. If you want a scenic Idaho trail ultra without extreme vert or a tight cutoff, this is a fair one to start with.
Yes. The 2026 course notes specifically flag it as a cupless event, so bring your own drinking vessel or hydration system to use at the aid station and start/finish.
There is one on-course aid station plus support at the start/finish area. Because the course is loop-based, how many times you pass it depends on your distance: once for the 10K, twice for the 25K, and four times for the 50K. Expect cookies, chips, sodas, water, and sports drink at each visit.
With no published elevation figure to plan tightly against, build your fueling around realistic finish time rather than a hard cutoff. Aim for roughly 60 to 90 grams of carbohydrate per hour and sodium in the 300 to 700 mg per liter range, adjusted upward if mid-August at Island Park runs warm and dry. Since the aid station offers sports drink and snacks rather than a full spread, plan to carry most of your own calories between visits. Use the free ultra fueling calculator to build your own numbers.
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<a href="https://runsummitline.com/guides/harriman-25k-50k-trail-run">The Harriman 25K/50K Trail Run course guide</a><iframe src="https://runsummitline.com/embed/race/harriman-25k-50k-trail-run" style="width:100%;max-width:420px;height:180px;border:0;" loading="lazy" title="Harriman 25K/50K Trail Run course guide by Summit Line"></iframe>This guide is independent and for planning only. The course details, dates, and entry rules come from public sources and can change year to year, so confirm the current specifics with the official race before you register or run. The fueling and pacing advice is general and not medical advice.
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