Snow18:
My advice is to simply be more active than you ever have before, both through aerobic exercise and calisthenics. Do not become a "couch potato" between now and I-day!
Aerobics: Alternate running with biking, swimming, jumping rope to avoid shin splints. Find a challenging trail, put 20-30 pounds in a pack, and go backpacking. Play basketball or soccer. IOW, you can have fun while getting in better shape, just be careful not to get hurt.
Calisthenics: In addition to full sit-ups, you can do "crunches" of all types - and you WILL do many hundreds of them starting on I-day
You can also do "planks" to also strengthen your core muscles. Definitely do lots of push-ups, too. Alternate doing them for speed versus trying for perfect form. You can have a parent, friend, or sibling (if you trust them) count cadence for some of your push-ups. IOW, go up or down only when they tell you to do so, and hold the 90 degree bent arm position until they tell you to go up.
The most important points:
1 Have fun while exercising
a lot
2 Avoid things that could injure you
If you can walk through the gates on I-day, your detailers will get you into shape, but Plebe Summer will be less difficult if you are in (what you think is) great shape.