The badgering by the 'drill instructors'.
According to them, you can never do good. Example, on the rifle range I had 8 bulls-eyes out of ten. The other 2 were just out of the inner circle. The D I (they hate that term) totally chewed me out for missing the 2 shots.
By the way, start exercising now. Long runs (trots), pull-ups, and push ups.
When you first start out (in boot camp) don't do all that you can, but make it look like you are. That way, later you can 'show improvement'.
It's a terrific experience and I wouldn't trade it for anything. But I would not do it again. (I was in for 4 and had it fairly easy because I was put into communications, top secret clearance and the whole bit)
Good Luck.
p.s. - once a Marine, always a Marine!