I'm in the Army, so I'll weigh in.
Combatives will be a lot of fun. It's a decent introduction to grappling. I did well in basic, so I thought I'd be good, til I went to an MMA gym. lol
It's a good foundation for starting MMA though... as far as cardio goes, it's not going to help a ton. When I got out, I could run 5 miles easily but a few rounds in, I was gassed.
One thing basic is really good for is building up your core. It helps alot in MMA. A good core will make you much stronger than you may look or for your size.
As for food, we had access to decent food... fruits and salads, etc. were usually available. The main courses weren't always the best, alot of fried stuff, etc. It doesn't matter though, you'll lose weight due to the incredibly long days (17-18 hr days).... unless if you are a bean pole, then you'll pick up some pounds of muscle.
As for PT, it's really not that hard, but that depends on what kind of shape you are prior. I'd recommend being able to pass the PT test before you get there.