The trick with interval training is to get enough long slow distance miles on the horse before you start speed work, and get the horse legged up enough to go a distance at slow speeds. Tom, himself, stated at one time to never go intervals with a horse that has less than 300 jog miles on him as that is the foundation you build on. Regardless if the intervals are a quarter, a few furlongs or a mile, you must have an equal interval or more to allow the horse’s heartrate and breathing return to normal for his jog speed; jog is only relevant to speed, not gait, can be slow jog trot/pace or very slow lope as long as the speed does not exceed 12mph or fall under 8,as I recall (never read the book, just hung around Tom years ago). The speed portion of the intervals is built up gradually to get air into the horse, and as training progresses, the last fast interval should be at the speed the horse races at, or within a couple of seconds of it. This method is a lot of work, as you can spend over an hour with each horse, but certainly does work well - you have tough horses, and they do last over a distance.