I was never military, but I was a police officer for two years. There's really no need for expensive gym memberships or classes in order to get functionally fit for career purposes. When I was going into the academy I researched what the standards were that I would be required to meet, and I started immediately doing those things in a similar way.
For me, I had to do push ups, sit-ups, a 300 meter sprint, and a 1.5 mile run. These are each graded pass/fail and based on age. I made a workout schedule that would be similar to the academy schedule (2 hours a day, 5 days in a row). I have an easy time with push-ups and sit-ups, so those were my Tuesday/Thursday workouts and I would just do those things in sets. Monday/Wednesday/Friday I did interval training for running to build up my cardio. I got in good shape quickly, and I was extremely well prepared for my line of work.
Personally, I'm a big fan of training with specific goals in mind and I think that makes it easy to see where you're at and how much further you need to go. I know each branch of the military has specific physical requirements. I'd strongly suggest training with those requirements in mind and incorporating any testable items into your workout routine so that you're used to doing them and will be able to do them well.
