But after pushing yourself at PT four days a week (atleast at my unit), you will start to fall out less and less. Your unit's PT schedule may be designed for maintaining a baseline rather than improving, so it's up to you to go running and do workouts on the weekend or twice a day if you need to improve.
Very well said.
Don't wait until you get there to start though, start early like he said. Stay in it.
Your previous question about the elliptical. Want to run faster and better? Running is the KEY. Elliptical, swimming, other cardio help and is good cross training. But running is the best because it is a skill.