What they need to do is shorten that distance to match the others. If tennis courts and football fields have standards for length, so should a horse race track
To go back to the grand slam comparison, that is part of what makes it such a great and worthy challenge.
In order for the horse to be considered among the elite it must be able to win the race at three different lengths.
In order to win a grand slam you don't just need to win 4 tennis tournaments in the same year. You need to be the best tennis player on grass, the best tennis player on clay, and the best tennis player on turf.
Who is a better runner: Usain Bolt or Wilson Kipsang?
You really can't say. Bolt is better at 100 or 200 meters, Kipsang at 26.2 miles. But if Kipsang could win a sprint or Bolt would win a marathon, then they would accomplish something even more impressive than the triple crown or grand slam.
What makes the grand slam or the triple crown such a great accomplishment is you need to be the best under a variety of conditions. Just like you need slightly different golf skills to win Masters than the US Open than the Open, than the PGA. And if you can win all four than you are truly the elite of the elite.
I don't think what makes the triple crown so hard is the horse needs time to rest as much as that the horse needs to be the best in a variety of lengths.