I admire your switch, I hope you can work things out. It took me a lot of fiddling to get to the point where any problems I have with discomfort are related to fitness rather than position.
I used to have low back pain and riding got rid of it. I don't think that 'bents use your low back as much. Eddy B (rip) used to insist that sit ups help lower back pain. I think this is one reason cyclists do squats.
I have gotten rid of hand pain for the most part through small changes in my position. A little longer stem and seat back a little.
I have never been sore at the end of a 1200k. Other distances, yes. I sometimes have trouble with neck soreness, but I am able to do shoulder rolls on the bike and that helps a lot.