I hear yer 'Big Guy'!
Ex techie myself and always battled (and so far won) against the scales but it gets harder every year as we get busier with less time and inclination to do fizz... anyway, when I went through I found that circuits 3 times a week plus workouts on the running machines and cross trainers, weights to finish off with and a reasonable diet were enough to prepare me for what we were about to receive!!
A lucky few on my course were of superhuman staus and found P.T. 'easy'. The rest of us mortals benefitted from the fact that it does not matter how fit you are on arrival, the PTI's are there to push you to new limits to see how you react. If your limit on a run is X then the staff will run you X+1. You will get fit....
It was mentioned above that there is a height/weight graph to get past. For groundcrew I used to just sneak past and that is what you must pass to get to go to OASC. To my surprise, once at OASC, they sprang a completly new aircrew graph at me where a particuarly peckish Ethiopian would struggle to pass on it. Obviously I failed it and was up in front the G. Captain doc at cranners. Imagine fit young chap (me!) who has worked towards this day for 1.5 years and played rugby/cricket all sporting life. converstaion went like this...
Doc: Come in...so you failed the graph...play rugby?
Me: Yes..and cricket (then I went into my best Charles Atlas pose)
Doc: Good lad..get out...hope you do well in the rest of your visit.
So its not so black and white! Quite a few shades of gey...
Start working hard towards it now...you'll feel the benefits quite quickly
PM me if you want any help!
best wishes and good luck from Vim