Something you may want to keep in mind, and I am no expert, is whether the horses are physically sound for the race and emotionally ready. It takes more than running around the track to produce a strong horse (and much more than the emotional and physically fit things I mention today).
Pat Parelli has been reported to have worked with Bobby Frankel a success American horse trainer in Thoroughbred racing. His work (from what I've read) had to do with preparing your horse emotionally for a variety of tasks associated with racing. As I discussed this time last year, the 7 games are easily found in the skills necessary for a successful thoroughbred from his activities at the barn to his journey through the finish line.
However, this year, I want to bring to light a study I recently read about the physicality of the horse (all breeds). It has been said for years that horses develop different based on breed but, this article, Timing and Rate of Skeletal Maturation in Horses truly questions that theory. Consider reading this article and then consider what you are asking from your horse. I can say that once I read it, I felt even more secure in the fact that I waited to start my horses until they were a bit older (actually, until this past year, they did not appear to be physically like an adult horse to the naked eye let alone to the radiograph). How interesting.