In the Army BFT we had pacers front and back ..as long as you finished ahead of the back pacer you passed ....is it the same in the Navy ? I know on the day with the build up most of the guys I did BT with, passed ..Good luck with it
Which Marathon and when ?, check out the course is it flat, hilly ? read up on any reviews that have been posted on this site...Agree with Cougie get yourself a training plan,this will help you to avoid injuries
If you fancy it, join a running club,you will find plenty of Marathon runners there who will also help with your training, advice etc..
Main bit is to enjoy your running and don't worry if you have a bad run, feel tired getting nowhere etc.. we all go through those stages