I can never understand why it is asking too much for a properly conditioned horse to run a mile and a half ever two weeks. I rode endurance in my youth. I did a 50 miler every other week with some 100 milers mixed in. These races usually ended in a sprint in the last 1 to 2 miles as the leaders contended to be first. Of course those horses were over 5 years old and were conditioned gradually over time. My training program began at 3 on a horse who was trail ridden regularly from 2 years old. I gradually increase speed and distance until we were going a minimum of 6 to 10 miles per day in an hour. We were able to compete when the horse was 4 in 25 mile competitive events. They were not races, but needed to be finished in 3 1/2 to 4 hours. The horse was judged on conditioning, using heart rate, respiration, soundness and attitude to determine the winner. Perhaps racing as a three year old does not leave enough time to condition and strengthen the bones and tendons? Unfortunately for the horse, muscles develop quicker than their supporting structure, but you would still think a three year old could handle 1 1/2 miles at a dead run, every few weeks.