Where do you live? That would help in giving a recommendation of a LBS.

Also, what is your height, bike inseam, and weight. Along with location this would help to recommend a used bike on Craig's List.

If you want to do a century and ride just on paved roads/paths, then you're probably looking for a bike with an endurance geometry.

Don't look at the Specialized Diverge, road test it. And road test several other brands of endurance bikes in your size. It might not hurt to road test a couple of bikes with more agressive geometries, just to see if you like them. And go to several LBS's to road test.

GH