Conditioning plan for older horse

My late teens TB tends to get muscle sore easily with dressage work. I would like to condition him on trails since he loves them and that will be his primary exercise now. We have trails on our property and there is one that is relatively flat, but the others have stretches of moderate hills. The guy that made the trails used a forest mulcher so they have quite a bit of wood chips that make it more challenging to walk on than it they were settled into the ground more. I’ve been hand walking him a few times a week on the trails and so far so good (but I have not been that consistent). Today I rode him with just a little bit of trot and kept it under 30 min. I’m looking for some conditioning advise so that he will not get too sore. I know about LSD, but I also don’t want to sit on his back for too long. I’m not looking to do any long several hour trail rides, but I would like to do some over the back trotting and cantering eventually for fitness.

ETA - he is the type of horse that will keep on if sore, so I cannot rely on him to tell me when he’s had enough. He lives out full time.

LSD is really a work/rest schedule which systematically increases the horse’s fitness.

Our rule of thumb is 3x/week work (with a day of rest between ideally) in which you increase either difficulty (ie hills, footing challenges) OR distance (say 20% increase, from 4-5 miles as an example) OR speed (from 4-4.5 mph average speed, or just increase trot to walk ratio a bit) each successive week.

Here’s an article on Rest that I wrote for Endurance News a few years back that might be helpful.

http://enduranceintrospection.com/wp/endurance-and-the-fine-art-of-rest/

Just take your horse out for a couple miles at a time and monitor how he feels. Use your best judgement on what you feel is best for him.