I have fallen over on the ground after 20 miles of running in both legs my quads and hamstrings were cramping. I got up and ran another 10 miles. Didn't have any choice, either run 5 miles back to the check-in table or 10 miles to finish the race.
This https://www.youtube.com/watch?v=CKWWRS9CpTY is being at your limits. But I'm willing to bet that a tiger would make her body remember that maaaybe there is actually (a lot) more energy left to keep going.