I always thought that was his greatest asset. Irony is, Hopkins had unbelievable stamina to back in the day, even at 36 he was able to use the ring for 12 rounds and outfox Tito, see the pace he was able to set against Glen Johnson, Mercado? Mercado was the native fighting in Equador, yet he was the one that got tired before Hopkins, and that was fighting at altitude, infact you might say that natural stamina is one of the reasons Hopkins is still around today. I believe every year beyond the age of 34 is significant, so even though Joe being able to work at the pace he could at the age of 37, is phenomenal, Hopkins stamina by that point, had deteriorated quite considerably given that he was over 40.