The majority of people who do "centuries" in under 5 hours are doing them in groups. It isn't impossible to do one solo. However riding in a group will not only keep the speed higher it will keep you fresher with someone to slipstream. Also the time just flies, before you know it, you're finished. On flat terrain with no crazy headwinds, under 5 should be easy. If the terrain is hilly then forget it. Somewhere closer to 5hr 30min is more realistic. The Pro's in the Tour Down Under, will do a rolling hill century in 3hrs 30min. Yep 45kph! That's something to aim for.

CHEERS.

Mark