Football Training (Redux)
In fact, I now know why the process seemed so familiar to me when I first began to go through with it. It's found in Dickson's Dorsai trilogy. All that standing on one foot until fatigue ('footigue'?) kicks in and then the other foot, and so on — the building of physical and psychic endurance through pain.
If I'd gone further down that path, I'd be doing handstands next, to strengthen my wrists.