I think Mikekayak hit part of it on the head- thigh support. Having done ALOT of long distance paddling, and having had severe soft tissue injury to my back from a car accident- following is my checklist for back pain would be:
1- Is the seat and seat back perpendicular to the keel? Do not guess...measure it!
2- Consider a hard back seat vs a back band. The additional support seems to help me.
3- Make sure both foot pegs or pedals are the same distance from the seat back.
4- Getting a good shoulder rotation when you paddle will strengthen your back muscles and reduce back pain.
5- Support the lower thigh directly infront of the seat, a chunk of closed cell phone will work good. You just want enough thickness to support the muscle until the angle of the leg going up to the knee lifts the muscle higher than the seat. The only warning is that after 2 or 3 consecutive 20 hours days in teh boat there will be some chaffing...