staying "fit" or "unfit" does not matter. Pure will and the motivation to do it is what really matters. Try conditioning your body and do things "parkour related" to condition your self.
Heres a video that may help you:
http://www.youtube.com/watch?v=kDCvAFwOqR4Remember these keys to parkour conditioning; Control, balance, flow, power, and Fluidity. Those are they key importance of parkour training. I does not matter "who can jump the farthest" or "who can run up that wall" its all about your body and how it works.