Four minutes and thirty-four seconds per mile. That’s the pace it will take to run a marathon in less than two hours. With the current world record at 2hrs 2mins 57secs, it means running more than six seconds per mile faster than any man has run in history. Many of those at the top of the sport – those who know firsthand what it feels like to push at the limits of human endurance – say it is unfathomable.