monkey bars are just three of the 200 challenges to overcome at the rat race dirty weekend. the largest obstacle course race in the world. this is a big objective to conquer and might be the biggest thing they ever do. reporter: two hours north of london more than 5,000 adults and children tackle a 20-mile course featuring 200 invented obstacles. got to know what to expect and what s next around the corner. can t retrain specifically for it and kind of why i like it. anything can happen in a 20-mile race. a mind game to overcome. push through the pain and get to the end. reporter: the challenge is daunting, but course designer jim carefully considered the race s degree of difficulty. obviously physically challenging. we don t want it so difficult it s what we call a suffer event. no joy in not completing something. my favorite touch is the little things we do. we have an old london cab people