People are mentioning running, but are leaving out that most of that running will be done in combat boots. Go out and get yourself the most uncomfortable pair of high-laced boots you can find (if you live near a military Exchange, get some black hot weather boots, not those nice comfortable desert boots) and run in them every day until December. You'll get blisters and such, but your feet will be used to the punishment by the time boot camp rolls around, and you'll be glad you put yourself through it on your own terms instead of theirs.
And remember, if you're not puking, you're not running fast enough. Seriously.
When I signed my papers they gave me no NDA really, other than telling me the whole "dont get in trouble or do anything illegal before you ship" document.