Welcome to Oklahoma's 2nd Annual "Ridin' Dirty" Endurance Series. This is the official USAC Marathon Endurance Championship for Oklahoma in 2009. If you like pain, sweat, dirt, and pushing the limits, this series is for you. Oklahoma has some of the most challenging endurance races in the Midwest... wicked, nasty, and awesome races that will test all of your endurance capabilities. Each of Oklahoma's 3 endurance races are held on three very different trails...fast, hilly, and technical. So it was a no-brainer when we decided to organize Oklahoma's 2nd Annual Endurance Series..."Ridin' Dirty."