If you want to do century rides in the fastest possible time then you'll want a dedicated road bike. If you're just doing it for the challenge of doing it then the cross bike is just fine.
I have three bikes for different purposes. My cross bike is definitely the most versatile. For fast road rides I swap my wheels and change the stem to slightly longer one and flip it down to get a more aerodynamic position. For long distances at a lesser pace I only change wheels, not as aero but more comfortable. For riding around town I use my 'tarck' road bike, it's fast and reasonably comfortable plus I don't have to worry about expensive components getting stolen. Usually I ride it fixed because it's fun but singlespeed is an option. Then there's the mountain bike for the trails that have too much gnar for the cyclocross bike.