I am seriously considering doing it. I'm just not sure how to effectively train for something like that.
At the Parks & Rec. track here, a 5K track, they have stations every so often where you can stop and do pull-ups, sit-ups etc. At my gym, there are a variety of classes that I'm in and out of. They've started incorporating more of a crossfit type workout. Not true crossfit but some of the same principles. They start with a lot of cardio work with jumping jacks, up/downs, punches...and then they'll run in groups out the back door and around the building to the front. There will be jump ropes, 10-20 pound balls that they squat with, small hurdles to jump...waiting on them. They run back through the gym and stop to do a variety of pull-ups. Back to the class room to start the cycle again.
A lot more to it but you get the idea. Constant motion using a mix of cardio, strength and flexibility training. If you have access to a gym with these type classes, that's as good a way as any to train for the type race you're talking about.