Much depends on the mental/physical make up of the horse.
When the horse in my avatar was 15, I spent the entire summer conditioning him for the Fall ride a couple of us wanted to do. It was 30 miles round trip in the Low Desert heat of Southern California. Temps can hit well over 100 degrees, out there, in October.
Coming home, my TWH and the QH's were staying close together at a good clip (running walk, trotting, no cantering).
The QH's started to slow down - both of whom were much tallker than my 14.3H TWH. Duke knew he was headed home and there was no putting him in a dog walk; we kept stopping for the other folks to catch up.
Finally they said to just let Duke go on as they could tell it was too frustrating for him to keep stopping and waiting.
He gaited the entire rest of the ten miles home. It was high 90's that day but the only sweat was under his bridle (I rode bareback), no heavy breathing, no signs of stress.
He is just a naturally/exceptionally tough horse that can handle more speed, for longer periods, than others. I have two other TWH's that not only could not hold a candle to him in a gaited race but could not hold a candle to him in endurance -- when they were all younger.
I'm sure you know people who seem to have more "get up and go" than ten other people, it's the same principle
With a horse like that, you have to always be on the watch for signs of stress because they will lie to you and say "I'm good" and maybe they really aren't but will keep going to the end, anyway.