well that really depends on where you are *text deleted* what are your times 3miler etc marine press ups etc ....but from when you apply you should be active and month before prmc is when you should be really training to get at least 21min for 3miler and atleast 40-50 marine press ups
If you train hard, you could get the results in a few months. Try looking at some fitness website they can give you a good programme for situps, pressups and running.
Ive just finished a 6 week programme for my pressups and i can get my 60 in 2 minutes. one hundred push ups
I found this website quite usefull.
Good luck mate!