Nevertheless, I think there's another dimension to the ability to endure. 2 summers ago, I hiked the 230 mile Cape Wrath Trail in the Scottish Highlands with my brother. I didn't have any of the physical issues that the author described (despite almost endless wet conditions underfoot and frequently from above too). I had trained for and finished an extremely hard 50km trail race in the Colorado Rockies 2 weeks before the hike started, and was in excellent shape even before we started.
Nevertheless, it was only my brother's inability to even imagine quitting that got me to the end. I was variously bored, confused about the why, and/or simply willing to call it quits. I wasn't in pain, I didn't dislike walking all day with an insanely heavy pack on, I hadn't even minded a 30' fall down a mountain. But it still took everything I had, plus more from my brother, to get me to want to keep going.
People who can do that easily have my respect, admiration and envy (like ... my wife!)