Hi,
I joined the Air Cadets at 14 years old. From experience, I can tell you that I was a little old, but I had a great time. Learned drill, polish boots, survival... All good stuff. I joined the reserves at 17 years old as a Clerk. The cadets helped me with the ranks, drill, and all that stuff on my recruit training. BUT I was not above everyone else. Most person joining the reserves do it because they really want too and most of them really work hard at learning all the little stuff during the training. I then joined the Reg Force after I got my College diploma.
My suggestion, look where you are from, there might be a communications squadron (reserve). They work with computers and communications, they should have someboday in the unit who will be able to tell you which trade are available.
For the push ups and stuff... That is easier than you think. Do 5 or 10 everytime you enter and exit your bedroom (for example) before you know it you will be able to do 25.
By the way, I am a girl too. And when you are 40-50 persons doing push-ups or going on a run everything seems much easier than doing it by yourself...
Good luck!!