Happily almost all endurance people regard these aspects of training as the most important part of a training session. There is almost nothing your horses cant do for you if you warm them up properly, protect them from overheating or chills during work and warm them down properly. The whole circulatory system including the kidneys need to be able to build up to the demands of the work and then to wind down and clear the blood of the products of heavy work efficiently. The single most important herb supporting this process is Rosehips. Physical soundness is very much in my domain.
Simple herbs can aid in the maturity of all ligaments and bones.
They certainly can dramatically speed the healthy and complete recovery from all injury large and small especially to the legs and hooves. There are specific herbal and homoeopathic substances which manage shock and assist in pain management. There are others, which can be carried on the ride and used as first aid to cuts and wounds on both horse and rider. There is also no reason you cannot support your horse's recovery from necessary surgical or medical procedures by using simple herbs. The herbs that I recommend one becomes familiar with in the areas of fitness and soundness are Linseed, Millet, Comfrey, Arnica, Wintergreen, Calendula, Yarrow and White Willow.