This was already asked I believe.
While it is fun to play Marine for a weekend, it's not required. It's there for your benefit and nothing else. Don't sweat it too much. The consesus is, you'll come back sore as hell no matter how much you work out. The basic gist of what you learn there is that, Your gonna be sore during OCS. No way around it unless your Chuck Norris. You take in a lot of information. Figure out a way to retain said information.
Oh, and if you puss out there...better rethink OCS. I went a few weeks back and we had 2 people drop out while the staff was introducing themselves, and atleast another 5-10 that fell out during the 6 mile ruck march, and that's cause our rucks weighed at most, 20 lbs...