Boot camp training is for those who are already fit! If you are not fit and able to do the training you are told to go away and come back when you are fit.
The biggest difference between real Basic Training and Boot Camp training is the massive pre-screening effort the Armed Forces does to ensure a higher rate of success. When you take a bunch of healthy men and women that are mostly 18-20 years old and subject them to high levels of training, they tend to respond positively. Young people have a greater window of recoverability. To further ensure success, the Armed Forces does extensive physicals on all enlistees. If you are overweight, you are instructed to “make weight” on your own before you can even start Basic Training.
When you watch some movie or TV show showing buff soldiers doing exercises while some drill sergeant barks at them, what you aren’t seeing are all the candidates that were rejected by the recruiter or medical personnel. In other words, the military stacks the deck in their favor by selecting the candidates that can best respond to military training. The personal trainer in the park isn’t getting those people.
The take away lesson here is that soldiers were ALREADY LEAN AND HEALTHY before they ever started Basic Training. And it wasn’t Basic Training that made them lean and athletic. For most soldiers it was youth.
What Michael points out is actually true for most sports and for a lot of what passes as exercise. Those who excel at it are generally fit already, despite their training.
This is something I try to get across in the Hillfit book, but Doug McGuff does a great job of tackling this in Body by Science too.