I don't do endurance racing, but have ridden horses in the backcountry for years and pushed them to their limits more often than not. Regardless of breed some animals are tougher than others and can handle the miles and weight better than others. They recover quicker and just hold up to work better even with the same feed and conditioning. I guess people are kind of like that as well.
We have a 25 year-old TW that still can out work most of our young horses. He is just plain tough. Load him up with a 200 lb man and 40 to 50 pounds of saddle, rifle, coat, saddle bags etc and he can still climb out of the deepest nastiest canyons faster and with less rest that most of our other horses.
My family breeds MFT's and we use different mares/breeding to produce different types of horses. We have one mare in particular that consistently produce horse with exceptional strength and endurance and another that produces exceptional movers, but they don't seem to have the same staying power. Depending on a riders goals one may prefer one over the other.
I could go on but my point is that I wouldn't give up on a breed you prefer based on an experience with a single animal.