An endurance horse is an elite, extreme, athlete.  If your horse is lacking luster in this regard do consider that it may be a nutritional deficit.   Calories in, calories out, what is eating your horse vs. what is your horse eating!  Check out these links:

Feeding the Endurance Horse
Source document: http://www.thehorse.com/articles/12424/feeding-the-endurance-horse

Feeding Endurance Horses Day to Day 
 Source document: http://www.equinews.com/article/feeding-endurance-horses-day-day

Hay for Horses: Alfalfa or Grass
Source document: http://alfalfa.ucdavis.edu/+symposium/proceedings/2001/01-061.pdf