Just my "one horse" study, but I have a TWH gelding that I used to ride A LOT. We rode all over everywhere, more than 50+ miles every week (he's the reason I started getting interested in doing endurance). This guy could easily go 25 miles in gait with me with hardly a sweat. I let a friend ride him one time. She weighed almost exactly the same as I did and was using my saddle. But, she didn't ride all that well. At the end of six miles, my gelding was sweaty and breathing hard. He seemed to struggle with the hills. For him, the rider made a huge difference.
