I recently started riding again to help get back in shape. Mountain bike on the road and trails at first, but have bought a used road bike now. I was hoping for a bigger difference in the efficiency of the road bike, but it just beat the heck out of my back. I have a nice fixed tail CHARGE 29'r that I think I will continue to use.
It takes quite a few hours of riding until your muscles adjust to the new format. Put in more time on it before you give up. My road bike is much, much faster and easier to cover paved ground on than any mountain bike could ever hope to be.
Finding the right seat and ensuring good fitment is key.