I have found a few things lately on Interval Training in my quest for ways to condition Kat for the marathon course. Pretty much all of them state that once your horse is fit, they are really, really FIT!
Article on IT in relation to race horses
A TB racing blog post discussing IT and different aspects including weight
For the most part, it is a strong workout, followed by a quick break, another quick strong workout, a break then push for another stronger workout, before bringing the horse down to relax for the day.
It is definitely something to consider as I prepare Kat for the marathon course for the CDE's, but also the one day ADT's. For the moment, I am focused on keeping him working at a strong trot and not getting lazy on me because he is bored. He is really coming along and every drive, proves to be a bit longer and encouraging.
It is also something for me to consider as well. We bought a new scale over the weekend. I am not impressed with the numbers. :(