There used to be a race across the USA, not sure if it was a one-time event, but the one I remember was in 1976. Some 6000 miles I think.
Various breeds competed, Quarter, halfblood, Arabs and even Icelandic horses.
Arabs came in first, Icelanders the second, AFAIK. I know some of the people involved.
Then there was a 100 mile race, where an Icelander won.
Also know a guy who won a 100 mile race in Australia.
The only thing I know, is lots of training, and in any case, NOT starting off too fast. The winner of the 100 mile race in Australia started off gently for instance.
I can search about for you if you like. What distances are you talking about?