Ummmm, now. Seriously, you want to be in incredible shape/have a habit of working out when you get there. The more fit you are, the easier EVERYTHING else becomes. Many of the basic workouts turn into stress RELIEF instead of added stress when you are used to doing challenging workouts.
I was a serious tennis player and in pretty good shape, but I added weight lifting and daily running starting with my final semester (so a couple weeks ago) and the altitude still was a pain (I am a girl as well).
Find a workout buddy, make it fun, and enjoy your senior year!
If you live at sealevel, begin preparing NOW for altitude: run, work out, run, work out, run, work out... you get the idea. Mine ran twice as far as they normally would have and were not too bothered by altitude when they showed up for BCT. One wore ankle weights, one decided not to do that. The latter had a little more trouble with altitude, but was acclimated in a few days.
The best time to plant a tree is 10 years ago. The second best time is now. But seriously, START NOW! You should try to get in the best shape of your life in the time leading up to entering the Academy. While BCT at every SA is tough, USAFA is an even bigger challenge once you factor in the altitude. Run, lift weights, practice push-ups. The better shape you are in, the better off you'll be.