The twentieth mile of a marathon is by many accounts the most difficult, the one people mean when they say, I hit the wall. The body’s glucose levels have plummeted, carbohydrate stores are used up, and some degree of dehydration is inevitable. Your legs get heavy; your lungs feel constricted; the small pains that you began feeling around mile ten — the heel blister, the inner-arm chafe, the aching hip capsule — elevate in pitch and volume until you become little more than a moving cacophony of distress signals. You aren’t yet close enough to the finish line to get excited, and you’re in far too deep to quit.