When conditioning for top performance you need to ride that fine line between enough and not enough exercise. Along with any conditioning program you must have a equal valuable nutritional program in place. You cannot get behind on the groceries or your whole plan will come to a screeching halt.
Most experienced horse people will tell you that they can tell you when their horses have lost a few pounds, if you think you are that good, go for it. I invested in a 1000 pound digital scale, 4' long, and have used it everyday for almost 3 years.
The trick is to weigh your horse everyday at the same time. Monk gets weighed before breakfast. He will hear me coming on the quad from the house and will pee and poop and walk to the stall door to wait for me. Remember if you horse just drank 4 gallons of water he is going to weigh quite a bit more. I would get a big calendar and slap in on the barn wall and chart your weight and maybe your resting HR..
On a training ride of some substance it would not be uncommon for a horse to loose 1 pound a mile. Now, most of that is weight loss is due to water loss. Now that does not mean that as soon as they go take a big drink of water that all of that weight comes back, it takes time. When you weigh your horse each and every day, especially if the horse worked the day before you get a very good insight as to what you have to do to keep up with the horse nutritionally. It can take a week to ten days for a horse to be back to full weight from a 50 mile endurance ride. Now I am not talking about a 8 hour 50, more like a 4 to 5 hour 50 miles....
I don't necessarily wait until full weight has been regained before I start them back to work. I use the quad to pony lightly or just go do some junk miles.
I am not a big fan of feeding fat, but you almost have to or you will get behind the curve, which you don't want to do. Do what it takes to keep the weight on your horse, some horses just cannot keep up with a accelerated exercise program because they are not good eaters. Monk and Ambrose get fed 3 times a day, all the grassy alfalfa they want and at least 3# of Omolene 200 plus 1# of other stuff plus supplements. On work days they tend to eat it all and other days not so much. They do get a post ride special mix to replenish muscle glycogen. I am trying to get then to eat more fiber, ie, beat pulp, with limited success, will keep working on it.