The Looniness of the Long Distance Runner, by Russell Taylor, a tongue-in-cheek tale of "an unfit Londoner's attempt to run the New York City marathon from scratch". It's hilarious.
I'd go along with Iron War too, plu7s our very own forumite pirate Andy Holgate's "Can't swim, ride, run", triathlon again, but it's his story of how he went from being a very overweight unfit unhappy guy with bad knees to an Ironman triathlete. A cracking good read and who knows, it may inspire you to dabble into triathlon too.