I'm painting my apartment right now, it takes a lot longer than I had anticipated, and it's fucking killing my back.
Bedroom is done, and I'm really pleased with the results. Now it's on to the most tedious parts, because I have to move my computer, sofa and tv out of the way. Not looking forward to messing with all the cables behind my computer
As for your back pains, it sounds that you might be doing a lot of bending downs while perhaps passing the paint roller, is that it? If it is, I suggest that if you can, you screw on a longer stick (normally a wooden broom handle) on to the paint roller's handle. At least this way you wont be needing to do so many bending downs, thus putting less stress on your lower back.
I'm glad I don't have to paint an entire house, because I'm sure it would be my undoing. As for my back, I have had trouble with it since I was 16. It starts to hurt like hell if I've been standing upright for a few hours, bending down actually relieves the pain.
It's odd, I should probably see a doctor
)