<< What is the recommended length of time between 50 mile, 75 mile and 100 mile races? Is the length of time impacted by the speed of the race? >>
It varies from horse to horse, it varies with age, it varies with experience, and it certainly does also vary with speed and difficulty of the race. If I am looking at 5-year-olds on their first season, I recommend no more than 3 or 4 rides, spaced at least 7 to 8 weeks apart, with a turn-out "vacation" of one to two weeks after each one. I pick the toughest rides I can find, because up-and-down terrain uses different muscles all the time instead of constantly doing the same thing over and over on a so-called "easy" flat 50. The difficult terrain also is more mentally interesting, and has the added benefit that there are more horses going more slowly, so the newbie is not demoralized when everybody runs off and leaves him.
An older horse on his first season, if he has some "life experience" for a base, may well be able to do more than this right off the bat.
By the second season, I like to see a horse go out every 3 to 4 weeks, and maybe move up a bit in his speed. One needs to be honest with how he comes off of each ride, though, and plan the next ride accordingly, instead of plotting out a season at the beginning of the year, and trying to convince oneself that just because he is thus-and-so age and has thus-and-so mileage, he should be able to do this schedule. Planning is ok, but be prepared to vary!!
By his third season, he should pretty well be able to do what he is going to be able to do, whether his maximal effort is going to be a middle-of-the-pack 50-mile completion or a horse building a portfolio for FEI. If you are diligent for three seasons and he is not the star you expect, he is not likely to be. If you are content with his level of achievement, love him and ride him; if not, get a better horse. Of course, be honest about how well you have done your homework; if your preparation left something to be desired, then there may still be more potential to be realized.
I don`t think the distance ridden makes as much difference as the speed of the race in terms of post-ride recuperation. Good, solid mileage horses go and go and go, weekend after weekend, but don`t run up front. People out trying to push toward maximal efforts need to give their horses more time off between (3 to 6 weeks, depending on the horse and the effort). There are certainly individuals that can consistently turn in sterling performances with less rest time, but they are the true superstars of the sport. One has to make choices most times between campaigning for miles and running competitive rides.
Heidi Smith, DVM--Sagehill Arabians (Oregon)