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. It takes place on January 31, 2026, and registration closes mid-November. The race is part of a series of trail runs organized to promote endurance running in a picturesque setting. Visit the organization's website for the most recent information.