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.
Going to disagree about the friesian mare, don't think her back is long at all. I think she's a nice example of the breed, but sounds like she's not in the mare book so as a breeding prospect (which you're not after) I would question. But they are pricing her as a broodmare. 10K for a trail horse I wouldn't pay but I don't know your financial situation, if you can afford it, more power. But agree that this is not an endurance horse. This is a 10 mile breed typically, always exceptions. If you want endurance, the walkers and large arabs are a better bet. Also agree with soenjer, quarabs are AWESOME. My boy is a quarab and can carry large men (i wouldn't do it to him all the time because I want him to live forever but...) and he can go for EVER without sweating. Find a large quarab and you might have a perfect match.