The Caldera was not a bad bike- but as you said- just a bit heavy. But many of us have done century rides on similar bikes with just a change of tyres to slicks. They don't work as effectively on the road as a proper road bike but the MTB gearing does work well on the hills.

Others have mentioned it--First find your LBS. You will then buy a bike from one of the Marques they carry- so make of bike is immaterial at this stage. What is important though is to test ride as many bikes as you can. That will give you an indication of what you ought to be getting- Road- Hybrid-CX or that nice shiny one on display with a price tag that makes you wince.