The canyon was cut by water that stopped running a long time ago. The rock has been arranging itself for a few million years and will keep going long after everyone currently alive has stopped paying attention.

None of which slows him down. He's running because the ground is flat here and because running is what you do when there's space. The scale of the place is a fact about the place, not about him.

Children get this right instinctively. You don't have to feel diminished by something enormous. You can just be in it.