I think one of the first, and longest lasting lessons I learned at Basic Combat Training, Fort Jackson, South Carolina, in 1966, was, when you think you can't do any more, when you believe you've given all you've got... if you really want to there's more inside.
Reach for it.
Not going into details, but, trust me, I've been at Nietzsche's point, "What does not kill me makes me stronger," more than once. I don't know if it made me stronger, but, what I had learned in Basic Training was what got me through it.
I realize there DOES come a point at which things such as the laws of physics, maximum velocity, and just plain time (my mother just died at 105), put an end to efforts and options, but, until then... buckle up buttercup and enjoy the ride.
Donovan Baldwin
Note: Photo is NOT me, but of a soldier in Basic Combat Training with the U. S. Army at about the same time.