Welcome to the forum full of marine hopefulls!.. Yup i joined for the exact same reasons!.. And i still dont know if my medical condition will be able to let me join!.. Im still hoping that ill be able to join!.. Going to ring the AFCO tomoz!..
I would say the first things you need to do before you can seriously think of yourself as a potential marine is get your 1.5 mile time below 11 mins, be able to pump out 40 press ups in a row followed by 30 good sit ups. Once you can do all that you have a good starting base to start your couple of months of serious fitness training before the PRMC.
Like Bradfordbulls said its not all about fitness, the other half is mental strength, determination, self will power, etc.
Try getting up an hour earlier than you have to everyday for a month, and use the extra time for a small run followed by a cold shower! This is really good for will power and mental strength.