Sean O'Brien 100K

Sean O'Brien 100K/50M/50K/26.2/30K is a trail ultramarathon event in Calabasas, California (1925 Las Virgenes Road). It offers five distances: 100K, 50 miles, 50K, marathon, and 30K, giving athletes a range of challenging options on scenic trail terrain. The race is part of the Sean O’Brien Trail Runs series and uses local trail networks for the course.

A notable feature is that the 100K is a Western States 100 (WS100) qualifier for 2026, attracting ultrarunners aiming for a WS qualification. Registration closes November 15 for this event scheduled around January 31, 2026. Visit the organization's website for the most recent information.