How about looking into quarabs? My horse that just passed away was a quarab, and he was a perfect, strong endurance horse. Before his semi-retirement due to a knee injury, when he was being ridden more often, he went on a fifty mile endurance ride (non-competitive), spring time in AZ, and he barely broke a sweat under his saddle. We rode all day. He was 15.2 or so, and easily carried my mom's ex-boyfriend who was over six feet tall. I've never had someone over 200 on him, but when he was fit, I'm sure he could have carried them all day. That's my suggestion- look for a well-crossed quarab. With the many variations in body type of qh's and arabs, I'm sure you'll be able to find the perfect one- as crosses, they can literally range from almost completely arabian-like to almost completely qh-like, in all aspects, build and temperament.