Try getting a training regimen down. Do 100 pushups and situps each day; I usually do mine in increments of 25. Try walking around more, buy an exercise bike, or just plain old jog. Make sure not to overdue it at first, though. You'll end up burning yourself out. Once you get your fitness up try to find a pickup soccer or basketball game. Both can be a good conditioning tool, because you jog quite a bit as well as sprinting.
If you've just been sitting on your ass, then your 'blockage' may just be due to inactivity, but there are other factors that play in. The above suggestion was geared more toward your joining the military. Also, once you get your speed/fitness up, try to hit the gym and lift weights 3 or 4 times a week. It will make basic training much much easier for you.
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