Why?
Why is it so impossible to accept that there are stories--even in foundational European tales, the very things that inspired Tolkien himself and thus 99.9% of D&D's milieu!--where there are people who explicitly, emphatically DO NOT use ANYTHING "magical", and yet still do things that should be impossible?
"Magic" is far, FAR more specific than "things that aren't physically possible with completely mundane skills." Magic means curses, or alchemy/potions/elixirs, or spells (which, in D&D, have mostly absorbed curses), or enchanted items, or explicitly divine "boons" or the like. None of those things describe Atalanta, or Odysseus, or Beowulf. They just, flat, don't.
The only conclusion one can draw, which doesn't contradict the explicit text of the stories involved here, is that you can have things that are not "magic"--they aren't any of the things that fall under the meaning of that term--but which are beyond-the-natural. That it is possible for someone's prowess or skill or speed to become so great, it manages to break the rules, even though none of the practice that went into developing that power ever broke the rules itself.
"Magic" is not the only thing that can break the rules of nature. Other things can too. Transmundane abilities are among them. Such explicitly non-magical breaking-of-nature's-rules is rife in fiction the world over, going back thousands of years to the very foundations of storytelling.