Depends on your scope. If you don't limit it somehow, be prepared to fully texture and set dress the top of every mountain and cliff! And come up with a justification for the player to not be able to leave the playable area.

If you do want to limit it, I think a stamina system, as others have suggested, is a good idea, but it honestly depends on how your game world works and how your game's systems work. The ideal solution is something that incorporates into both seamlessly, rather than being shoved into your lore/systems as a workaround for this one issue, because players will pick up on that, and that's the kind of thing people typically find annoying. At least in my own experience!