I made the mistake of not preparing myself physically for basic training. The only thing I had a huge problem with was the run, and that's the Army's two-mile. (Ya gotta understand, I'm just God-awful at running, always have been). It was quite a struggle for me to finally get that thing.
Maybe AFTER Army basic training (like right after) I would've been in good enough shape to be able to enlist in the Marines with some confidence, but I dunno.
Personally, if I could go back to high school (*SHUDDER*) knowing what I know about myself now, I would've gone into the Air Force for an active duty hitch, instead of joining the Army Nat'l Guard, probably. I could've gone aircrew and pursued my first love, aviation. But that's neither here nor there.
Here's a good place to go on getting in shape.
The Navy SEAL Workout. It was put together by SEALs to prepare people who want to try for the SEAL teams, but it can work for you. It starts off slow and easy, and builds up week by week, and includes pushups, situps, pull-ups, running, and swimming. If you can stick with this you'll be in some kind of shape, let me tell you.
MAKE SURE, though, that you want to do it before you sign up. IT CAN GET YOU KILLED. Falling on the field of battle for your country is an honorable way to die, and may sound appealing (at least, I've met people who think so). Dying isn't the only thing to worry about. How about getting your legs blown off? Becoming paralyzed? Blinded? Given brain damage or driven insane by nerve agents?
I bring this up because you have to consider it. It's not a decesion to be taken lightly, at least, not if you go into combat arms.
But if you do go into the Marines, you have my respect, anyway. Don't let that sway you one way or the other, though. Just give it some thought.