What a great decision that you made to join the Army! You definitely have bravery within you to make that choice. With that bravery, I think you have it in you to keep going even when you feel like giving up. Because, no lie, cardio training is hard work! And there are so many times in exercising when you feel like throwing in the towel, but in the end, exercise is well worth improved health, longevity and better energy levels when you get older. I'd suggest you see a doctor and ask what you should do to get in better shape, what your goal amount and intensity level of exercise is and how soon you can get to training with an injury or if there's anything you can do to heal it faster. Keep in mind too that you won't see results immediately and that exercise should involve a balance of high intensity to build speed and strength and low intensity to build endurance. Also, remember that flexibility will help prevent injury in the future, so stretching will be highly beneficial. You can do it!
Just because everything's changing
Doesn't mean it's never been this way before
All you can do is try to know who your friends are...