I don't know anything about endurance, but I have a 21 year old Arab who is still doing low hunters and dressage with my daughter.
I have no doubt what you're proposing is possible, but the big question for me would be the time investment to condition this horse properly, and the possibility that you discover along the way that it's just too much for him. Are you willing to risk investing in this horse knowing that you may hit a wall? Our Arab used to jump and do very well at dressage, and he can still do those things, but he'll never compete at top levels because of arthritis in his hocks. We only discovered this issue when we started working him harder and harder. Horses are stoic, and they won't show pain sometimes. Our horse was still able to jump, but would pick up the wrong lead on the same side frequently and this was new so we had him Xrayed and sure enough, both hocks were pretty well fused. We have put him on Previcox and he's doing better, but we do find that he gets tired after a few classes at shows, so we take it easy on him. My daughter adores him, and won't hear of another horse, and as long as she's ok with just doing low-level shows, it's fine with me, but he would not be able to compete at higher levels.
Your horse may be just fine for endurance, but you may find that after some long rides, he is showing signs of fatigue. This is something you probably won't know until it happens, so if you go ahead with this project, keep in mind that you may have to call it quits at any time and maybe consider having a back-up plan.