Any horse can compete. They have had ponies compete in the Tevis Cup! If you are riding endurance to finish as long as your horse is conditioned you can compete. Now riding to be competitive is a different story. But an OTSB should do just fine. I am riding a chunky QH. I know I will never "win" but an endurance motto is "to ride is to win". For me, just finishing a ride in the specified time and vet out clean will be a MAJOR win for me.
It is a dream of mine and I don't want to give it up. I will never be competitive but I am more than ok with that. I love trail riding and riding strictly for pleasure. I do enjoy the training part with my horse. I enjoy monitoring his heart rate on the trail (yes, I am a MAJOR GEEK) and tracking miles on my Garmin.
Read all you can get your hands on (I bought a book "Go the Distance") and learn about heart rates, electrolytes. I know several endurance riders here locally and they are a fun bunch of folks!
I think for now just finishing would be a major accomplishment and then maybe go competitive with more experience under my belt. Haha, I like your motto! Who is the author of that book?