Army basic training in 1974 was pretty tough, and I got kicked in the butt (literally) a few times, but it wasn't anything like this (and was about 3 weeks shorter, too, which makes a big difference when you're just trying to hang on).
You just have to respect anyone who can go through all this and survive reasonably intact. That is the essence of the Marines---they can take anything, and will. Recruits start learning about that reputation right away, and have to live up to it or they're out. They have to learn how to suffer without whimpering.
It's worth bearing in mind that most boot-camp inmates were children just a few years earlier. Most of them are not yet fully mature. So the basic-training experience is a real shock to the system. Recruits confront many realities for the first time, and the environment they're in makes those realities especially harsh. They do have the advantage of being more physically and mentally adaptable than full-fledged adults. But they are less capable of putting what is happening to them in context.
The novel surprised me. On first picking it up (my wife brought it home in a box of books from a thrift shop), I didn't think it would be nearly as good as it is. It's written well and unpretentiously. It poses interesting moral questions and explores them thoroughly. The main characters---two drill sergeants and two "maggots"---have depth and are intriguing. And, the novel is entertaining and keeps you turning pages as the tensions build.
One of the better books I've read in a while.