I'd think you would be fine if you condition him slowly for LD. I got my girl at 13 and started doing LD style rides with her this year (she is 14 now). She is a champ and sure she could do endurance if we conditioned a bit more. If she were to show signs of not enjoying it or it being too much for her we would stop. As it is now Blaze sees the trailer hooked up she gets excited and if I let her loose she would run & jump into her spot, she loves going placed so much.
I also just got a 17 year old gelding. He too is ouchy on gravel. But he is also under weight. I have no plans to push the old guy into anything he doesn't want to do. I've noticed as he had gained weight he has shown more spirit and energy. Once he is up to a good weight we will stretch our rides a bit longer. My hope for him is to be able to do 10-15 mile rides with walk/trot so that my daughter can ride with me when I am working my girl. I don't expect him to be able to do 30 let alone 50 miles....but he could surprise me yet.
All in all it really comes down to the horse. No matter what you do as long as you both enjoy it, that is the most important thing.