Sean O'Brien 100K

The Sean O'Brien event in Calabasas, California, offers multiple trail running distances including 100K, 50 miles, 50K, marathon, and 30K. The 100K race serves as a qualifier for the 2026 Western States 100. The course features scenic trail views with notable vantage points around miles 7 and 15, providing runners with beautiful natural surroundings.

This event is well-regarded for its challenging terrain and supportive atmosphere, attracting ultrarunners aiming for qualification and personal achievement. Registration closes mid-November, and the race takes place in late January, making it a key early-season trail running event. Visit the organization's website for the most recent information.