In J. R. R. Tolkien's Lord of the Rings there are countless tales of exceptional physical endurance. I can accept most of them. The three-day ultramarathon of Gimli, Legolas, and Aragorn across the plains of Rohan (~135 miles) is reasonable enough (see TwoTowers, 29 Dec 2002). So are the long, desperate marches undertaken by various of the hobbits. Challenging, yes. Impossible, no --- given enough time, water, food, and hard work.
But in LotR there's one crucial feat that gets little recognition, but which I find absolutely incredible: the climb up the Endless Stair by Gandalf and the Balrog, from the lowest dungeon of Moria to the highest peak of the mountain Celebdil, aka Zirakzigil.
Yes, neither Balrog nor Gandalf were human. But to climb even a few flights of stairs is exhausting enough. To race up more than a mile of elevation gain? In hot pursuit of (and/or flight from) a deadly enemy? Without stopping for food or drink? I don't think so!