Health apps are a big category for iphone, ipod and ipad apps.
there are loads available, some work and are useful, some dont and arent.
ive tried hundreds of them, and here are the ones I think might be of use to you

nike+
http://www.apple.com/ipod/nike/
iphone / ipod appp
tracks the distance you have gone.
you need a pair of nike shoes with a slot for the nike+ device thing, and a nike+ device thing.
or a normal pair of running shoes, a stanley knife and the nike+ device thing.
useful if your primary means of exercise is walking. not useful if your primary means of exercise is xbox.