Check out RUSA for a full explanation. It's not a race, but a self-paced timed event. You get, for example, 40 hours to finish 600k. You have to follow a specified route, and reach a few checkpoints on the way within a (fairly wide) time window, and have your card signed at each. You must not receive any help between checkpoints. Any time under the limit is a winner, and the prize is fun and pride.