They never tell you how far it is.
At the end of Delta Force selection, after weeks of solo land navigation in the West Virginia mountains, sleep debt, and a rucksack that only gets heavier, candidates are handed a map point and told to move.
It's called the Long Walk, and it's roughly 40 miles over steep, broken terrain with a loaded 70-pound rucksack, done alone, with no announced distance, no remaining mileage, and no published time standard.
At each checkpoint a cadre member confirms you arrived and gives you the next point. Nothing else. They don't tell you how far you have to go or how far you've already gone.
The course was built from the British SAS endurance march Charles Beckwith brought home, and the cruelty is the information gap: you keep walking until someone tells you to stop.
What the Long Walk selects for is the willingness to keep moving when the finish is invisible, the feet are wrecked, and quitting would be rational.
Delta Force needs the right kind of operators in The Unit, and the Long Walk reveals who they are.