Sorry to say this , but if your vet is giving the OK to train and compete, why don't you ask HIM for a conditioning schedule?
Your first post, and possibly others, have made it sound like ypur horse has a tendon injury(possibly resolved), arthritis(which requires injections), BUte for pain, is 20 yo,and the vet said 10 miles MAX.....
If you think HF members are protective of the horses, wait until you get to an endurance ride. If you mention ANY of this casually at a ride, you may be VERY unhappy with the response you get.
And since an endurance ride is FIFTY miles, not 10, you have a LONG way to go to get in condition for that.
You need another horse to do endurance, there is NO Novice level.....