How much could one walk without stopping before death or exhaustion claimed them?

I’m reading a book by Stephen King Richard Bachman where the plot consists of walking…One hundred teenage boys have to walk a large distance, keep up four miles an hour, and never stop. Soldiers are following all boys in vehicles, and when someone slows down, stops or whatever, they get a warning. After three warnings, they are shot dead by the soldiers.
(the warnings go away after an hour, if no more have been accumulated in that hour)

I don’t remember how many miles they need to walk, but it’s like 400 or something.
So obviously, a lot of them die. Some faint, some get cramps, some stopped too long, some get heat strokes, one guy got shot for ignoring three warnings when he tried to fight another dude. Some guy died trying to take a dump. Stopped too long too many times, got shot.
Anyways. It’s bloody terrible. Never thought a book where people just walk the whole time could be so intense.

But what I’m wondering is, how realistic is this? Given the details of the book I gave here, how long could someone walk nonstop before the body can’t handle it anymore? Nonstop is the keyword here.
I realize it must depend on the person, their health, what shape they’re in, their age, what footwear and clothes they have. In the book, they get food every 24 hours, which is just paste in a tube. They get water whenever they want it.

So, in real life, how would this all work?