I dont know if you've made your decision yet, but like others before me I'd encourage you to write out your reasons for enlisting and alternatives. I've done a tour on active duty army and while it was not hell it did have its down sides. Homosexuals were not well tolerated, but there were a few I've served with and they kept it lightly under wraps and no one gave a damne about it, while others were living in hell because they made waves. Thats an important concept. Dont make waves and you will likely be alright.
Check out other options such as the coast guard and Air Force. I was Army and stationed on several Air Force bases and let me tell you they were MUCH more lax about exercise standards and they had much much bettter living standards.
In the Army you do what they tell you to as far as excersize goes. You will bulk up a bit in basic training but if you have a thin physique and high metabolism you'll get out in better shape and not bulk up too much. When doing excersize its good to concentrate on your PT test as thats what they obcess over. Run at a good pace, do push ups effectively, and do sit ups effectively. Sit ups are harder on the guys, push ups are harder on the gals. Work on building endurance more than strength. High reps with low weights can actually build up bulk, so focus on resistance training with moderate weights to build endurance without overdoing it on bulk. You can lose some bulk over time, if you watch it. Running is a big deal in the Army, and it does not require large bulk. Ace your run times and you will likely be thinner and still score better than the bulky guys who have no problems on their pushups.
Also, reconsider Infantry. While Infantry is the Queen of battle, they dont tolerate Queens well. It's a rough world. If you have half a brain try and go for something more mental than physical. Smart soldiers are harder to come by, and in the end we are all infantry. The medical field is a good goal, or military intelligence (but you will have to pass a security clearance - dont lie as its a sure 'FAIL'). Supply is also a good all around field, its vital to any mission and you'd be surprised how important and involved it is. My father (retiered Army) once told me that if you make only one friend, make it the supply sergeant.
Hope this helps.
