There’s a poignant anecdote in Alex Hutchinson’s book Endure where he describes the race in which he drastically reduces his 1500 meter running times as a collegiate runner. He had, for some time, been on an unsuccessful journey to break a four-minute mile. Spoiler alert, Hutchinson goes on a stretch to run several PRs in the 1500, but eventually plateaus. “Reaching the ‘limits of endurance’ is a concept that seems yawningly obvious until you actually try to...