Everything you need to know before you toe the line at Fullen Road — the terrain, the sections that will break you, the ones that will save you, and exactly how to run all 31.4 miles of it.
The Run the Rim 50K is brand new for 2026 — the first time the Arnold Rim Trail Association has offered an ultra distance at this event. That matters. There is no race history to draw from, no Strava segments with competitor times, no worn-in race-day groove in the trail. You're writing the playbook.
The course runs the full length of the Arnold Rim Trail and then some — point-to-point from Fullen Road in the south all the way to White Pines Lake in the north. It plunges in and out of three Sierra drainages before climbing to the ancient lava cap ridgeline that forms the Arnold Rim at 4,464 feet. The forest you'll run through — Ponderosa Pine, Sugar Pine, Incense Cedar — has been growing since before California was a state.
The elevation numbers look manageable on paper. 5,724 feet of gain over 31.4 miles averages out to a gentle 182 feet per mile. Don't let that fool you. This course doesn't average anything. It drops hard, climbs hard, rolls brutally, then drops again. The first mile alone loses 700 feet. The section from mile 18 to 19 gains 600 feet in a single push to the highest aid station on course.
"Starts out with an aggressive downhill and the hills never let up. 22-plus miles of mostly singletrack with 4,500 feet of climbing and 4,000 feet of descent."
That's the race director's own description of the base ART route — and the 50K extends beyond it. Arrive with respect for the terrain.
You start at the Fullen Road trailhead at 3,288 feet, shuttle in from White Pines Lake at 8:00 AM. The trail immediately commits. ART-Lone Madrone is the steepest trail on the entire course — a 12.7% average grade that opens with a 700-foot descent in the first mile. The trail is mostly wide singletrack with a rugged, ungroomed feel. Roots, loose dirt, berry vine encroachment on the edges. It does not warm you up. It tests you immediately.
You'll bottom out near San Domingo Creek — the first of the three drainages — somewhere around mile 1.2 at roughly 2,569 feet, the lowest you'll be all day. The creek crossing can be technical depending on spring runoff. By late May the water is usually passable but expect wet feet. After the crossing, the trail reverses: a sustained climb back up to 3,330 feet at Helispot Aid Station.
After Helispot the course enters the San Domingo section — the most remote and technically demanding stretch of the day. The ART-San Domingo South trail drops you into the second drainage, a 3-mile descent on overgrown, narrow singletrack with creek crossings and aggressive poison oak on the margins in late spring. Stay on the trail and don't grab anything you don't recognize.
The San Domingo Zigzags are exactly what they sound like: two miles of relentless switchbacks with no let-up. This is not technical terrain in the rock-scrambling sense — it's just steep, sustained, and demoralizing if you're not mentally prepared for it. The grade data from the actual GPX shows repeated sections at -20% to -25% descent. Your quads are already compromised from Lone Madrone. These zigzags are where a runner who went out too hard begins to unravel.
At the bottom of the San Domingo drainage you'll hit Cowell Creek. Another crossing. By mid-morning in late May this is usually knee-high at most, but the footing on the creek bed is slippery. Take 10 seconds. Don't twist an ankle at mile seven.
"Lots of switchbacks and creek crossings. Beginning to get overgrown. Long pants would be advised this time of year."
The ART-San Domingo Headwaters is four miles long and takes you from the creek drainage up onto the ancient lava cap that defines the Arnold Rim. This is where the trail character shifts entirely. The overgrown, jungle-feel singletrack gives way to exposed ridgeline running with views west to Mt. Diablo and east to the Sierra crest. On a clear May morning, this section is spectacular.
It is also demanding. The Headwaters trail climbs from 3,594 feet to nearly 3,900 feet over four miles with no flat relief. The surface transitions from forest duff to volcanic rock — the lava cap is literally ancient hardened lava, and the trail surface reflects that. Expect embedded rock, uneven footing, and sections where you're picking your way rather than flowing.
San Domingo Hillcrest at mile 12 to 13 is a brief respite — the one section of the course that qualifies as genuinely rolling singletrack. Run this. Take it as a gift. The climb to Cougar Rock begins shortly after.
Miles 15 to 19 contain the most sustained climbing of the entire race. From Sunset Aid at 3,888 feet you'll climb to Cougar Rock Aid at 4,197 feet — gaining 309 feet over four miles, but not evenly. The ART-Cowboy Loop section has the highest single-segment elevation gain on course (683 feet) and that gain is front-loaded into a brutal push between miles 17.5 and 19.
The real GPX data shows grades repeatedly hitting 25–30% on this section. These are hike-your-ass-off grades. Do not try to run the Cowboy Loop climb at mile 18. Nobody is watching. Power hike it, stay relaxed, keep moving forward. The runners who blow up in the final 12 miles of this race almost universally go too hard here.
The views from the Cowboy Loop ridgeline are the reward. At 4,197 feet you're standing above the entire drainage system you've been fighting through for the last 15 miles. On a clear day you can see Sacramento to the west. The cathedral forest of Ponderosa and Sugar Pine north of Cougar Rock — the section you're about to run — is genuinely beautiful. Take three seconds to look. Then get moving.
North of Cougar Rock, the trail changes its personality completely. The Cowboy trail and Wilma's Wiggles are smooth, fast singletrack through the cathedral forest — Ponderosa and Sugar Pine overhead, soft duff underfoot. The grades are still real (the GPX shows rolling climbs throughout the 20s), but compared to what you've been through, this feels like flying.
This is where you get your legs back. It's also where the race can go sideways if you respond to the runnable terrain by going too hard too soon after Cougar Rock. Give yourself two miles to reset after the high point before you start pushing pace. Then, from about mile 22 to mile 29, you can open it up. The trail is mostly ridgeline running with occasional punchy climbs, and the views — when the trees break — extend across the entire Central Valley.
There is one more significant climb in this section: a 400-foot push from mile 27 to mile 28.3 that catches a lot of runners off guard. By this point in the race your legs are cooked. The climb is not as steep as Cowboy Loop, but it comes at a time when everything hurts. Don't panic. Power hike it. Run the descent into Equine.
Bamm-Bamm is the final trail on course and it's a 5,033-foot-long stretch of singletrack that parallels Valley View Drive toward White Pines Lake. The trail has some fun rock rolls and minor technical features — unusual for the final miles of an ultra. Stay focused. The rocks don't care that you've run 29 miles.
The final descent to White Pines Lake drops you from 3,950 feet to 3,882 feet — a gentle 68 feet over the last mile, with the lake visible through the trees. The finish at White Pines Lake Park is a grass area adjacent to the lake. Pizza Factory feeds you from 11am. Dogs are welcome at the finish, which is the right call.
"Enjoy the sandy beach on White Pines Lake at the end of your journey."
You've run 31.4 miles through three Sierra drainages, across an ancient lava cap, through cathedral forest, and back down to the lake. The race is new. Nobody has run this 50K before. Whatever time is on the clock when you cross the line — that's the course record.
Park at White Pines Lake Park, 1965 Blagen Rd, Arnold CA — not at the start. A shuttle is included with your entry and takes you to the Fullen Road start. Check in opens at 7:30 AM, 50K start is 8:00 AM sharp. Don't miss the shuttle.
No crew access at most aid stations for the 50K — the remote San Domingo section makes vehicle access impossible. Crew can meet you at White Pines Lake for the finish. Plan to be self-sufficient between aid stations.
9 hours total. All runners off course by 5:00 PM. The 9-hour cutoff works out to roughly 17:13 per mile — generous for fit runners, tight for anyone who struggles on technical terrain. The hard section is miles 0–15. If you're on pace through Sunset Aid at mile 15, you will finish.
Arnold sits at 4,000 feet. Late May mornings start cool — expect 45–55°F at the 8:00 AM start. Midday temperatures on the exposed lava cap section can reach 75–80°F. Carry sun protection and plan your layering around a 25-degree temperature swing through the day.
This is not an easy 50K. The marketing calls it "a legitimate introduction to mountain ultras" and that's accurate — but introduction doesn't mean gentle. The terrain is technical, the San Domingo section is exposed and overgrown, and the Cowboy Loop climb at mile 18 has the potential to end days that started too fast.
It is also a beautiful race on a trail system that deserves more attention. The Arnold Rim Trail Association built something special here — 31 miles of maintained singletrack through a Sierra Nevada ecosystem that most California runners have never touched. The fact that this is the inaugural 50K distance makes it even more worth showing up for.
Run the first five miles like you have 26.4 left. Because you do. Respect the San Domingo section. Power hike Cowboy Loop with confidence, not desperation. Fly on Wilma's Wiggles. Finish at the lake. That's the race.