There are muddy events for everyone — from 5K to 12 miles, timed and untimed, and somewhat difficult to super difficult. All are very, very dirty. Some of the most popular series:
Tough Mudder: 36 races a year, 10 to 12 miles long. Untimed, team-focused events founded by Will Dean, former counter-terrorism agent for British Special Forces. Upcoming California events: July 7-8, Snow Valley Mountain Resort at Running Springs; Sept. 22-23, Truckee; Sept 29-30, Patterson. http://www.toughmudder.com.
Spartan Race: 41 races, 3+, 8+ and 10-12 miles long plus a 48-hour challenge. http://www.spartanrace.com. Upcoming events: June 30, Midway, Utah; Dec. 1-2, Malibu; Jan. 26-27, 2013, Vail Lake Resort, Temecula.
Warrior Dash: 48 races, 3 miles long. Upcoming events: March 31, Lake Elsinore; April 28, Florence, Ariz.; Oct. 20, Henderson, NV.; Oct. 27, Hollister, Calif. http://www.warriordash.com.