There's an ongoing thread about this in the Coaches Q&A. Basically, the consensus is that they're stupid. They're not even really races; they're paying to jog/walk from one location to another for the privilege of standing in line for the opportunity to participate in an adult version of something you'd find on a playground, only you get cold, wet, or electrocuted while doing it. You'd be better off going to Disneyland. If you want to complete in something for time or a personal record to see what you're capable of, you'd better off sticking with strictly-running events, where the crowd itself isn't an impediment to your progress (most of the time).