Disclaimer 1) I am in the Army, not the Marines. Disclaimer 2) I'm not even in Basic yet. However, since I'm going in via the National Guard, I am in RSP, which is a weekend thing that is meant to prepare you for boot camp (IMHO every branch should have this.)
With that in mind and with what experience I've had so far I can tell you this: BE A PT BEAST.
If you can run, do push ups and shoot you are going to have drill sgts thinking that you are squared away. The guys who are physical studs in our training platoon very rarely get in trouble and when they do it is always minor. Guys who are academically great - no one cares. But the PT beasts get a lot of compliments and atta boys from the training staff.
Make sure you can do at least 60 push ups without stopping before you hit basic. 80 is even better. Same for sit ups. And be able to do a two mile run in less than 14 minutes. (I think the marines might have a longer run. Army is two miles. Look that up.)
If you're a PT beast, and you pay attention and keep your mouth shut, the general word that I get is that you'll be fine.