Keep in mind that, if Secretariat's 1-1/2 mile time on a dirt track remains not broken for a very long time, it will be because virtually no tracks in the U.S. sponsor races at that distance. Yes, the Belmont Stakes is still run every year at 1-1/2 miles in New York, but the track cushion Secretariat ran on in 1973 was several inches thinner (i.e., faster) - since then, the running surface has been made much deeper (slower) to help protect the horses' legs.
It's really no different than when Man O' War set the world record for 1-3/8 miles in the 1920 Belmont Stakes. His running time on a dirt surface lasted as a track record for some 71 years -- solely because horses stopped running races at that distance on dirt tracks many, many years ago. If no one is running the distance, then who is going to break the record??
Secretariat's 2:24 running time has been bettered a number of times -- before and since 1973 -- on grass.
Also, although visually exciting to watch on video re-runs, when put in a proper perspective, his 31-length winning margin is not very meaningful. Except for Sham, who broke down in the early stages of the race, and was eased up into last place, the running ability of the three (3) other horses that Secretariat faced in the Belmont Stakes was unremarkable. Secretariat had it all his own way because he faced no Spectacular Bids, no Seattle Slews, no Affirmeds, Dr. Fagers, Buckpassers, etc. If he had, no one would be talking today about a 31-length margin of victory.
Even as it is, 31-lengths is virtually meaningless as a true indicator of the race result -- consider, if the other three (3) horses had fallen down at the start of the race, one could then silly-claim Secretariat had won by 1000 lengths. In either event, given the conditions of the race, and lack of ability of the other three horses, exulting over a 31-length winning margin or a 1000-length winning margin is meaningless.
One comment about Sham -- many supposed "expert" commentators who should know better, seem to delight in stating that Secretariat ran so fast that he broke Sham down before getting through the back stretch. Keep in mind their running time at 6 furlongs (1:09-4/5) was seriously fast for a 1-1/2 mile race, but not for a 6 furlong running time -- that time is several seconds slower than what the top stakes horses in the U.S. are capable of. Secretariat didn't stop Sham. Sham sustained an unfortunate injury (it happens every day to good horses, including Barbaro and Go For Wand) that caused him to be eased and then pulled up. If Sham had been able to stay in the race, Secretariat was not going to win it by 31-lengths.
