What do they look like now? Are they in good shape or a little on the thin side?
Our trail horses go about 100 miles a week in the late spring and all summer. They stay in good shape and several are near 20 years old. Last summer, they kept working right through the 100+ temperatures without losing weight.
We feed them a custom made grain pellet and they do great. They get 5 pounds a day of it when they are being ridden. The few tougher keepers I have get a cup of oil added. All get free choice grass hay and some grass, but our grass hay is pretty mature.
Obviously, a lot of grain in not good for obese horses or sedentary horses, but horses that work hard burn it up and do great on it. It has been the standard for hundreds of years without horses having any problems if they are working hard.
I feed some alfalfa in the winter and spring, but I do not favor it in the summer for hard working horses. They have to drink a lot more and they sweat a lot more, so you have to be sure you can water them several times a day and not just once a day during the day like we usually do.
We also wear out a full set of shoes every shoeing all summer long. Our big horses are shod with SX 8s with clips because they are thicker and do not wear through at the toes. [They are 3/16 inch thick.]