101, get fit some months before you go to OASC. Get a pair of boots and break them in. I'm sure I looked a right 'prune' all those years ago running around the Coltishall back roads with my gangly legs, a back pack and my new boots. But I was thinking of my future and didn't give a toss what anyone else thought. I followed the running with half an hour in the gym exercising to build up my cardiovascular (sp?) rhythm thingy. I was never going to be strong, but my increased stamina really helped with the outward bound stuff on ITC.
Set yourself targets within a timeframe, and stick to them.
Good luck!