It took all the way until big john's and inkandpaint's posts for someone to clearly say that it might be the wrong size of bike. Unless I missed something, anyway..

kage65, assuming that the saddle is at the right height for proper leg extension, that bike may be too big for you. Maybe not, but the old "fistful of seatpost" rule for bikes with horizontal top tubes could apply here. If it's indeed too big, you'd have to stretch out just to reach the bars, and that would make it really difficult to get a bit of bend in your elbows like big john says. Then your arms are too rigid, and your neck would droop below your shoulders, which I'd bet is the cause of your upper back pain; if it were lower back pain, I'd bet that the bike was too small instead (because that's what hurts on me first if a bike is too small).

bikerjp made a good point, too. It was either him or another BF member who posted about lowering his bars to alleviate upper back/neck pain. In that case, getting his torso lower ended up with his core muscles supporting more of his weight, taking it off of his arms and shoulders, which in turn didn't make him droop below his shoulders (or, in other words, hunching his shoulders up).

Get fitted. I'm like inkandpaint in that I'd choose a drop bar bike every time for long distances. I even converted one of my bikes to drops because I wanted to use its rear rack to carry stuff.