A dear friend of mine went through the roughes training we have in Norway, and he became a "Marinejeger". I think that's what we call it. So he used to tell me stories all the time of what happened, what they had to go through. And It'd often be a lot of cool stuff along the lines of this.
The best trained guys, people being elite level in sports, boxing, rowing, taking medals in the olympic etc, a lot of those would be "super" and sick strong the first 3-4 days, after that they would be the first ones to break, cry, give up and go home.
During endurance tests, it would be the hardest for the people with the least, but after everyone got pushed to their limit and beyond, it just went on mood and willpower, then it shifted completly.
They first had to stand in a line, then give away all their belongings, sign some papers, and then helicopters came and flew them off. After that they got transported to an Island, there they all had to strip naked, then they were checked for "hiding" stuff in cracks and whatnot.
After that, a new set of clothes, and the best meal you can imagine, and warm beds.
In the middle of the night it starts, the two months of "hell". Alarm goes off, they all have to get up, get taken to the sea, and thrown in the sea, there they had to complete some test, swim down and through a ring at night.
Every day the first week would be a new test desgined to break out, and around 200 people is trying to trim down to 20 or so.
The last weeks of the training, they get put out in a large forest/mountain area, and they have to survive for a few weeks, crazy stories about finding cows, hugging then at night to keep warm, eating berries, stealing pie's from farms and whatnot lol.