[Bring_Back_Shantz]

One thing I would suggest, is that whatever the contest is for, if the focus really is on overall health/fitness, then the prize shouldn't be for the person who loses the most, or whatever, that's just a good way to discourage people and make them quit.

Instead, you'll likely do much better if you make any prizes draws where everyone who stays in the competition for the whole time is elligable.
You quit, no prize, you stick with it, you might get it.

That way you could still do something like have everone sign up for a run and whoever completes the distance, regardless of time, gets in the draw.