Just as a note for conditioning, there is such a thing as over conditioning that will ultimately wear your horse down more than is necessary and will lead to injury or a shortened career sooner or later. My vet said that we needed to be putting in over 20 miles each week in order to get my boy in proper conditioning, but both he and the Arabian Horse Association say not to push your horse over 30 miles a week during conditioning. Obviously, when you go to a ride you're going to go over that, but in general you should stay under 30 miles each week. Here's an excerpt addressing this in an article from the Arabian Horse Magazine about preparing your horse for their first endurance ride and keeping them in shape (It's a really great article for anyone and their horse who wants to do endurance!):
But don't get tempted to ride more than 30 miles a week or work your horse every single day.
"The biggest mistake you can make is to ride five or six days a week without a day off in between rides," says Loving. "With older horses, once they are in condition, riding them twice a week should be sufficient provided you make those workouts count. Riding day after day without rest causes a horse to break down from too much wear and tear.
You can read the rest of the article here: Welcome to Arabian Horses.org - Marketplace