Hi.
There's no reason to go from a MTB, all the way to a "racing" style bike ... unless you see yourself getting into the local racing scene.
There are plenty of options in between -- from hybrids and tourers to "audax"-style bikes. These allow for wider tyres and the addition of mudguards and racks.
They will generally come w/ a wider range of gears, as well. There's nothing to be gained from grinding out a high gear when lower gears are available. Spinning a lower gear is kind to your knees and generally more efficient.
I have a range of bikes, but the most practical for most uses (including long distances) sports 42mm tyres! I don't even have to think about bumpy roads with those and they are very fast as well.