I can tell you one thing, a career in the fitness industry requires a good amount of maturity and plenty of experience in dealing with people. You'll also need to know just about every exercise there is, have a thorough knowledge of anatomy and nutrition as well. There's a lot of learning you need to do inside and outside of the gym, and there's a lot of growing up you need to do. Don't finish highschool, take a weekend certification and apply for a
personal training job at a gym - don't be one of those idiots.
Having said all that, don't go into the Army thinking that because of PT you'll come out an exercise guru. Yes, you'll gain invaluble experience that you won't get anywhere else, and yes, you'll stay in shape, but if you want to get into the exercise biz be sure you hit the books as much as you hit the weights... if you can get a degree on the Army's tab, by all means DO IT!
Though I've never been enlisted, I think the military is an honorable career path and great experience even if you just do your 4 years and be out.