It can be done, but itıs not easy. a. Use indoor arenas to work on your flat work and basic dressage (getting an endurance horse in the right, rounded frame is a big help to him on the trail. b. Ride early in the morning before going to work (even if you start in the dark, it will only get lighter, and horses have good night vision. If you and your horse know the trail, you can move right along). c. Schedule long rides for the weekends.
if your horse is reasonably fit at the end of the fall season, it doesnıt hurt to let him rest a couple months, then resume real work in early february, when the days are getting longer again (and the worst of the weather is presumably over).