I suspect that is true for many difficult physical goals.

My dad told me that the first time you climb a mountain, there will likely be a moment not too distant from the top when you would be willing to just sit down and never move again, even at the risk to your own life. Even as you can see the goal not far away.

He also said that it was a dangerous enough situation that as a climb leader he'd start kicking you if he had to, if you sat down like that and refused to keep climbing. I'm not a climber myself, though, so this is hearsay, and my dad is long dead and unable to remind me of what details I've forgotten.