I dunno if this will help but I had a similar issue on those kind of rides. Being in that position for so long was actually causing a shortening of my ab muscles and then off the bike they would cause a pulling in my lower back. Solution was to lay down over a large exercise ball (back on the ball) and stretch out the ab muscles really good after, before and whenever the back starts to twinge. That has done wonders for me. Mine got so bad I could barely walk at times and this simple stretch did the trick, go figure.