[ConorMacNessa]

They should breed for strength in the bone before speed. Maybe some blood from British horses bred for the steeplechase is in order. Then train them for endurance as well as speed.

[Mamzelle]

A three-yr-old horse is too young to run with weight on his back for these distances and after heavy training. Breeding is not going to change that. Thoroughbred racing also uses two-yr-olds, which is begging for a breakdown. This is just my own opinion from hanging around horses, and even the occasional racetrack. There are a lot of horse people who would agree with me. Most horses simply cannot thrive and break down. It's a sport rather like boxing.

You suggest steeplechase blood? Instead, check out the average age of a steeplechase horse.

If you have a horse you want to keep for a dozen years, you start with only the lightest exercise at two years old. You gradually increase the demands--you don't want to injure those bones and connective tissue if you want to still have your friend around after ten years. But these high-strung T-breds? Run 'em till they drop, sell the geldings and hope the mares will last long enough to breed.

[Salamander]

There is some validity to the point.
Look at this photo of the great Man O' War;

Physically he looks more like an Irish warmblood than a 'modern' Thoroughbred. Today's racers have the "greyhound" build which does allow for a greater stride and speed but tends to sacrifice general bulk and bone density.