Opposite way here - had a road bike then bought an MTB, although that's set up with an 'aggressive' position I did have to fit bar ends to get comfy !
I've had to get used to flat bars again for commuting.
Road bikes take some time to get used to - your back will suffer at first. I started doing some miles again in December 2006, and for the first 3-4 months of getting back into it, my back would last about an hour before pain set in.... that's gone now and I can ride for hours in a race position !