Well, I'm joining the Air Force in November, so I know how you feel. I'm also not very athletic, so getting in shape is really difficult, but I've created a plan. Hopefully it'll work for me, and maybe it'll work for you.
1. We're out of shape. Don't run every day, run every other day, because when you're building new muscle in the legs or arms or whatever, you need a day of rest after the excersize time to allow the muscles to repair themselves.
2. Do what everyone else said, eat right.
3. Vary your workout, and try to get as much stretching and low-impact aerobics in as you can, because that's the kind of excersize you can do every day. The flexibility will come in handy later in life as well.
4. Stay on your schedule. Don't let yourself slip. It's all too easy, and believe me, more difficult to stick with a plan then to stop after a week because of the progress you've made and undo all that you've done.
5. Try to make it as fun as possible. Make fun of the video aerobics instructors. Play music while you're jogging/running. Have your cat/dog in the room while you do pull-ups or lift dumbells.
That's all I can do for you. Good luck to you in Military school. Good luck to us both. ^_^