I wouldn't compare it to Marine boot camp. Probably not even close. You'll definitely have long days and may not get very good nutrition during the day (pack your own lunch), but other than pulling some G's, I wouldn't say it's incredibly tough. As you move on to bigger and better things that may pull even more G's (if you track fighters), the more physically demanding the flying will get. In primary training, you'll do basic aero manuevers requiring 3-4g. Maybe 5g. Anything more and you're probably pulling that much on purpose. In the beginning, 4g will feel like a ton of bricks on your chest, but you'll eventually get used to it. If you're in decent shape, you'll be fine. There are plenty of fatties in UPT who get through fine.