A big aspect of a joining the military is fitness. Every branch regularly tests their members in physical fitness tests, usually push-ups, sit-ups, and running. So if you are not someone who works out regularly, make sure to start before leaving for basic training. Get used to running a couple of miles at a time. Work yourself up until you meet the standards for physical fitness training. It will make basic training easier for yourself and reduce your chances of being held back for not meeting physical fitness standards. And yes, you can be held back from graduating. That's a little tidbit recruiters "forget" to tell you.