i snapped 10 years ago on the underground i was absolutley furious that i was paying an extortionate amount to be transported worse than cattle would be so i took my hobby and turned it into my commute too. never looked back and only rarely use the underground, now i can make my £10 oystercard last a month!
My main reasons are enjoyment and health, followed by economics, althouth Peyote definitely has a point!
In terms of health though, I've found I've put on weight since I began commuting in earnest this year. I have lost the balloon that was my distended gut but I have leg bulges now that weren't there before, and they're clearly heavier than the gut was!!
Admittedly I do eat like I am the Pig King of Antioch though.