This trail makes for a challenging run due to its elevation gain, but it's doable for fit runners. The grade is never brutal, just steady.
Though this trail shows up on maps as a loop trail, to create a full loop you'll have to conclude your run on a series of roads, making it not an ideal looped route. The most common use of this trail is to start at the trailhead off CA 88, near Carson Spur. (Look for the parking sign on this map) Most visitors work their way up to the Thunder Mountain
summit, which offers incredible 360 degree views with a surprisingly low amount of effort on the way.
That most commonly traveled section climbs steadily to 9300 ft, and you'll feel you've gotten quite a workout. But the climbing is never brutal - just steady. The ascent has tons of scenery to keep you busy. You'll start climbing through forest, but along the way, you'll see hillsides covered in wildflowers, dramatic views of Two Sentinels (8780 ft peaks) and the dark volcanic north face of Thunder Mountain
Continue up Thunder Mountain Spur Trail
to the summit for spectacular 360-degree views.
You can tack on and additional section at the start, by starting further southwest on CA 88, at the true start of this trail, and you can continue further on this trail past the Thunder Mountain
summit, but most agree that wouldn't be worth it.