I've found that gallop/canter interger training works great for cardio. Get into a nice canter and hold for five minutes (more or less depending on his condition). Bring him into a gallop (a bit beyond a hand gallop, but not full out... 60 or 70 percent). Hold this speed for a minute, then bring him down into a steady canter. Repeat several times. You know your horse; you should know how much he can take. As an endurance rider, you know about the limit -- the threshhold between not pushing hard enough and over working an unconditioned horse. The limit is different for each horse in each stage of fitness.
After this, have him stand perfectly still for 10 minutes. Take his pulse. Focus mostly on pulse; respiration has a lot more to do with heat than fitness (not saying it doesn't have anything to do with fitness though!) If he is down near resting (8-12 beats per 15 seconds), perfect. You can push him a little harder next time if you think he's ready.
If he doesn't recover and is over 14 beats, wait another 5 minutes. If he isn't at resting by than, you pushed the cardio just a bit too hard. Not a lot too hard, just a bit.
If he's over 17 beats on the first count, you're going to hard. If he's over 17 or so after 20 minutes, waaaaaay too hard. Dangerously hard.
Last edited by Brighteyes; 01-18-2012 at 10:44 PM.