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
I have the same kind of spasms if I am on my feet all day at work and not able to move around (doing anesthesia for a 2+ hour surgery has me crippled as well, even if I have a stool to sit on). Movement is my back's friend.
The smooth motion of a gaited horse is still enough my back counts it as non-movement and gets angry. So I am all about posting!