Having just got into some regular longer rides recently (30+ miles), I have been asking a similar question myself. And my conclusion...?

A CX bike!

My current mountain (style) bike does the 5 mile commute pretty well, but starts to get uncomfortable after about 45/50 mins. I wanted to change to a road bike (Trek 1000 looked like being the one), but there are several non-paved section of my commute and the longer ride is mainly on an old cycle path with breaking-up asphalt. So i have now turned my attentions to something which i see as basically a toughened up road bike - a cyclocross bike!

In case you are interested, the leading candidate is the Specialized Tricross Sport (cheapest model). Kona Jake the Snake came second only on price grounds and i wanted higher spec components than comes on the standard Jake.