Run, run, run till the day is done. If you used to be a runner, then it will all come back. Your muscle memory will kick in and you'll be on your way.
Build up your upper body strength.. push-ups, pull-ups, overhead presses. I think the Navy utilizes the modified crunch for their PT tests. Avoid the temptation of lifting weights. Coming in big/muscular won't do you any good. You want to focus on endurance and stamina instead. You can mix things up. A couple days, do distance runs. On other days, do daily dozens, mixed with wind sprints between exercises.
Go swimming. A lot. Also learn how to do survival swimming. They'll probably teach it to you anyways. But basically, it's just learning how to float/swim on your back for extended periods of time, in case you go overboard.
If you're going through a recruiter, see if they have any all hands PT days coming up. It should give you a better idea of what your current level of fitness is. And where it should be.