Love that NSH, looks like a serious yah mule there.
Depends on where you ride, lots of breeds can have the endurance for endurance, the problem comes with heat management. That's wher eyou run into problems with QH and Morgan crosses. And also My TWH. Heat, Those big butt muscles hold heat. Higher heat, needs more water, which leads to more dehydration and higher pulse rates. My walker mare runs like a champ when its 60 or below, temps start climbing and so do pulse down times. That being said a close friend that runs walkers went to a florida winter ride, temps dropped like a rock, lots of Arabs were cramping up at the holds, shivering and stiff and not moving out. She won event with no issues.