mainly driving 120-130 most of the way.
As for back pain... none.
Actually sportbikes are considered more confortable as per DOT vs custom bikes... mainly because your back doesn't suffer, while road imperfections get absorbed by the arms and legs (sportbikes) vs butt + back (custom).
Our bodies were designed to absorb shocks better through limbs rather than our spine through the butt....

I tend to drive a lot... i do from 80,000 - 100,000 yearly, where half is leasure driving w/ no destination for sure.
First summer on the bike, did 45,000 on it

edit: all units are metric (of course).. that's the standard