Exactly! Johnson was in the ring for nearly 2 hours ,and fighting for 78 minutes, at his age and in his shape he was not going to win a marathon fight, against a durable giant.
JOhnson was able to fight at the top level a lot longer than Willard, from memory. So, there seems no doubt that there came a time when Johnson Willard II, even under those conditions would likely be JOhnson's fight. If we are assuming an immediate rematch, i think that JOhnson shouldnt be dismissed. For a start, i think that the Leonard Duran effect would have to come into play. Willard living life up for 6 months after the win, as teh hero of the country and Johnson definitely not underestimating Willard, refinding his will to win at all costs and improving his stamina or even timing, now that he knows Willard has such a great stamina and chin. I think JOhnson, who would never change too much, would dominate a rematch and if required take less chances and not wear out or allow Willard to wear him out.
I just dont see Johnson winning a rematch and here is why: Johnson, by his nature fought a very reserved style. He was always laying back, clinching, minimizing the action and then punishing guys on the inside or the way in. Against Willard, and particularly in a contest over 45 rounds he couldnt do that. Willards massive size advantage allowed him to lay back and make Johnson come to him. By forcing an over the hill over weight man to force the fight for 20+ rounds in a style that he wasnt comfortable with he allowed Johnson to break himself. By the midway point of the fight, so the early teen rounds, Willard is steadily increasing his work rate and landing long, hard jabs to Johnson's body. In the second to last round you can see Johnson is exhausted and struggling while Willard is fresh as a daisy and has grown immeasureably in confidence. There is no doubt in my mind that Willards knockout was legit and no doubt in my mind that Johnson is not winning a 45 round contest against Willard. I think Johnson was enormously strong for his size and that advantage was used to full effect against the smaller men he fought but while he may have even been stronger than Willard (who was reputed to be strong as an ox) that difference in strength wasnt so great that it allowed Johnson any huge advantage. Whereas Johnson had a navigate a mind field of long punches to even reach Willard and to do this he had to use quick bursts of speed of both hand and foot, very tiring. It was an excellent game plan in reality but Willard was simply too big and too durable for it to work over such a long fight. I dont care if Johnson got himself in top shape. At that age and in that era being in top shape was not the same as it was when a guy was 25 years old. He may have taken Willard 5 or even 10 round more, and may have continued to hold his lead for that time, but even if Johnson lasted 10 rounds more against Willard he would still have 9 rounds to go. Thats a daunting thought and not one I would bet against.