You get sore tired backs from walking your horse for 20 miles? Perhaps it's because when I do distance riding I'm not actually "riding" the whole way. I stop to allow for some grazing and if it's multi day camping ride I'll lead for awhile to give the horse a break (it is after all carrying it's tack, my gear, my food, much of it's food). Leading it for a mile after grazing gives the horse a bit of a break from my weight at least and weight is the killer for distance riding.
I use ride our TW to town (over 30 miles round trip) and free time permitting I'd take 2 and 3 day weekend riding treks generally between 40 to usually under 80 miles for a round trip camping weekend (one of the great joys in my youth). Never noticed any pain (easier on me than backpacking
). Was thrilled to have that smoothing riding stallion after doing it on QH's before we got him. I could walk a QH and cover a little over half the distance I covered on our TW and would feel the same. The only issue with our TW was he hated to trot so making him trot was a lot of work. Not a problem though, because I wouldn't trot him and I was happy to let him do his fast walk which left me no more tired than walking on one of the QH's. After a week of riding a QH with the cows (that stallion was never going to make a good cow horse) he was a wonderful relaxing ride.
His fast smooth gait is the only thing I find lacking in my current horses, but when I was shopping for my latest two I couldn't find a TW that met the other requirements I was looking for (I'm sure they're out there, I just didn't find one). So I gave up on getting a gaited horse and accepted that to cover the same amount of ground I'll be back to posting and being more tired at the end of the day or just travel a shorter distance. Doing up to 10-15 miles still doesn't give me pain, but I haven't done a 20+ mile day in at least 20 years. The only bright side is I'll get to condition myself for doing the longer distance riding again while I'm conditioning my new horses for it.
Would make my day if the younger one ended up with a smooth pace a couple of the Saddlebreds I've ridden had (the older one likes to jump like some Saddlebreds), but that's only slightly more likely than her flying