Well the point is humans can run(although not at a dead sprint) for 24+ hours without stopping for breath as you suggested, without slowing down significantly assuming they pace themselves reasonably - it's certainly possible for a human to run at a typical Bethesda gate for more hours than you'd likely ever continuously move in the game.
These points you're making are still very absurd. It's very nice that some humans are capable of achieving that endurance, but I would find it utterly ridiculous for every NPC encountered in Skyrim (including the elderly) to not show any signs of fatigue whatsoever after running a good distance. Even athletes who play sports today show obvious signs of fatigue after just a few hours--walking around with their hands on their hips, taking large breaths, perspiration, etc.
You know what I'm talking about. In Oblivion or Fallout an NPC would just suddenly stop and mechanically revert to their relaxed animations as if they were robots. Or if they ran up and initiated conversation they would instantly freeze and talk without any heavy breathing (though I suppose the awkward-zoom-style conversations of those games is partly to blame).
The point is, not every single NPC can be the epitome of human endurance and for them to, once again, not show any signs of exhaustion will, once again, make them look ridiculous and unrealistic.