⏵ Course guide · HPRS random-loop timed race
The Human Potential Running Series' Endure Trail Runs at Mueller State Park is built around a genuinely different format: the 12-Hour event has you draw colored balls to determine each loop you run next, alongside fixed 10K, Half Marathon, and Marathon options. I will walk you through exactly how the format works first, then give you a pacing and fueling plan built for a long day at Mueller. There are free calculators along the way to dial in your own numbers.
These facts come from the official Human Potential Running Series registration listing. Check the current year details before you commit. Race logistics change year to year.
HPRS calls the Endure 12-Hour "one-of-a-kind," and the mechanic backs it up. There is no single fixed loop, instead, your race is assembled in real time.
Each runner pulls colored balls from a bucket, and each color corresponds to a specific loop within Mueller State Park. You run whichever loop you drew, return to the staging area, and draw again, for up to 12 hours. Your final result is the total mileage you covered, not a fixed distance you had to complete, and you are free to stop participating whenever you choose within the window.
If a randomized course is not your idea of a good time, HPRS also offers a traditional 10K, Half Marathon, and Marathon at the same event, all on Mueller's double-track and singletrack trails with zero pavement, sharing an 11-hour cutoff.
Because every loop in the random-draw format returns you to the staging area, you get repeated access to the aid station no matter which combination of loops you draw. That is a real advantage over a point-to-point ultra where aid gaps can stretch for hours.
You cannot memorize splits for a course you do not know in advance, so pacing here is about managing effort, not chasing a predetermined number.
Since you will not know which loops you are running until you draw them, build your pacing around a sustainable effort level rather than a rehearsed splits sheet. A grade-adjusted pace target still helps: it gives you a real sense of what an honest, repeatable effort feels like on Mueller's terrain, whichever loop the draw hands you.
Because you can end your 12-Hour effort whenever you want, decide in advance what would count as a good result, whether that is a mileage target or simply finishing the full window, so fatigue at hour 8 does not make that decision for you.
⏵ Free tools to pace this event
Frequent returns to the staging area mean fueling here is about consistency, not carrying enough between rare aid points.
Aim for roughly 60 to 90 grams of carbohydrate per hour early in a 12-Hour effort, and expect that number to taper as the hours build, particularly if you are out for most of the window. Because the loop-draw format brings you back to the start regularly, you can adjust your intake in real time rather than committing to a big single load.
Sodium in the 300 to 700 mg per liter range covers most runners at Mueller State Park's elevation. Use each return to the staging area as a checkpoint to reassess rather than sticking to one fixed number for the whole day.
⏵ Build your fueling plan
Get a carb, sodium, fluid, and caffeine plan per hour built for your weight, your goal duration, and a long day on Mueller State Park trails 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 race-day plan built around YOUR fitness, whether you're chasing distance in the 12-Hour format or a set time on the 10K, Half, or Marathon. Summit Line reads your real training, builds a plan for sustained trail effort, and rehearses your fueling so race day is something you execute, not guess at.
It is not a fixed course. Each runner draws colored balls from a bucket, and the color determines which loop within Mueller State Park you run next. This repeats for up to 12 hours, and your final result is however many total miles you covered, not a set distance you had to complete. You are not required to run the entire 12 hours either, your result is simply the miles you ran within however much of the window you participate.
Alongside the headline 12-Hour random-loop format, HPRS offers three fixed-distance options at the same event: a 10K, a Half Marathon, and a Marathon, all run within Mueller State Park's network of double-track and singletrack trails with zero pavement. All three fixed distances share an 11-hour cutoff.
Because the format keeps you circulating past a central aid station rather than sending you deep into a remote point-to-point course, you have frequent, predictable access to refuel. Aim for roughly 60 to 90 grams of carbohydrate per hour to start, adjusting down as the hours accumulate and your gut tolerance shifts, especially if you are out for close to the full 12 hours. Sodium in the 300 to 700 mg per liter range covers most runners at Mueller State Park's elevation. Build your starting numbers with the free ultra fueling calculator, then adjust by feel.
The 10K, Half Marathon, and Marathon distances all carry an 11-hour cutoff. The 12-Hour event is a timed format rather than a fixed distance, so it simply ends when the clock runs out and your result is the total miles you completed by that point.
The course network combines double-track and singletrack trails through Mueller State Park near Divide, Colorado, with zero pavement anywhere on course. Because the 12-Hour format draws from multiple loops rather than repeating one, you will see a genuine variety of Mueller's terrain over a long day rather than the same mile-long stretch on repeat.
The random loop-draw format is actually a friendly introduction to timed racing: you are never locked into a fixed distance, you can stop whenever you choose, and the variety of loops keeps a long day from feeling monotonous the way a short repeated loop can. The fixed 10K, Half Marathon, and Marathon options with an 11-hour cutoff are also a generous, low-pressure way to experience Mueller State Park if the full random-draw format sounds like too much of an unknown for a first attempt.
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<a href="https://runsummitline.com/guides/endure-trail-runs">The Endure Trail Runs course guide</a>This guide is independent and for planning only. The course details, dates, and format 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.