I never understood why the Triple Crown was such a big deal until I watched a video about it recently.
Apparently, when mammals exert themselves, lactic acid builds up. Humans can get rid of this acid quickly, but horses take a long time, like 3-5 days. So the three races of the Triple Crown are all well within the rest period. That means the horse that wins all three races never truly recovered from the first race while running the second, and same with the third. Above that, its running against fresh horses that haven't run in weeks.
All of this just makes Secretariat's run that much more amazing.