Marciano is a good mention, but he can't touch Armstrong's pressure and workrate. Do you really think that Amrstrong couldn't go 20 rounds?
From the films I have seen of him, jumping up and down, shadow boxing, and doing sit ups, all this AFTER he has gone 15 rounds !!! yea 20 no problem. keep well.
Say what you will about his skills, but Juan Diaz was freakishly well conditioned. Daniel Zaragoza was also a machine, him specifically being more impressive due to his advanced age (for his weight class). Joichiro Tatsuyoshi would also qualify - he was a freak for training.
Marciano is more impressive imo due to his size and weightclass. Same with Frazier. There are others you can compare to Armstrong around those classes for stamina and / or workrate. Not really so for Rocky and Joe. Rocky was also a bonafide power puncher, which is massively energy draining, yet he threw nearly everything with it.
I mean, I haven't seen him, but John L Sullivan had some freaky endurance and toughness when he was on form.
That doesn't mean he got stronger every round as the fight went on, something only Marciano and frazier did. But just like Eddie Futch said: " Under Cus and Rooney Tyson was well conditioned and ready to go all night, lots of gas, great combinations, great defense. Under that parade of clowns King brought in, well, no defense, no conditioning, no combinations, if the knockout doesn't come by six, out of gas."
