When I went in the Old Corps..per say many years ago. They used to be able to kick your azz into shape. I think the hardest for me was the running in the beginning. If you join, don't smoke and you have when I was in do at least a three mile run, twenty pull ups and I think 200 sit ups in three minutes. We had a confidence course to run as many times as your Drill Instructor wanted, you will walk no where. Good luck.
Fmr. Marine Sgt. Scout Sniper