Don't bother. They'll already have your school fitness records,
so failing isn't going to do a thing for you.
Neither's trying anything else to get out of it. At my interview, a
sargeant asked me, "So, do you want to join Commandos?". Trying to
give him a bad attitude, I asked him, "Do I have any choice?". When
he answered, "No", I said "So that the f*** are you asking me
for?".
A week later I got my letter stating that I was in, and I spent the
next two and a half years finding out that my attitude was exactly
what they were looking for. Go figure. 
Look, one way or another, if you're slated to go, there's not a
whole lot you can do about it. From my experience, go in, get into
it, learn a thing or two and have a little fun - otherwise,
regardless of where you're posted, it's going to be a waste of
time.
