It just seemed neat. After James brought up increasing anaerobic threshold on the 400m thread, I got to thinking about how Secretariat had a huge heart (literally). I tried to find something about it on the internet, and I found this.
They mention the difference in development of the heart in humans depending on the type of training they are doing.
For what it’s worth, everything I’ve seen about training horses indicates they go short to long.
They all go short to long. Look in the form guide and check out their recent racing history. All lead up races, for longer distances, the gradually move out in distance. Training is the same.