What is REALLY top human Endurance?

You're probably remembering the reference for stat advancement, where they use She-hulk as an example. They point out she started at Amazing Strength, and through natural advancement is now at Monstrous, but to justify rising above that one CS increase would require some kind of excuse, like further mutation, exposure to gamma rays, etc.

Hmm... well... perhaps, it warrants reading from cover to cover again...

There is ONE human with an Incredible that I can think of. Kingpin has an incredible endurance, though that may be due to body mass ... that any shock, damage, or the like is dispersed by such a large physique.

Endurance also encompasses being able to shrug off damage. Your typical marathon runner, while in great shape, tends to be slender. So that probably brings the average down. A soccer or football player runs and takes hits, then gets up again for the next play.