I've only completed a handful of flat centuries and none recently.
What I do recall, is make sure you keep a schedule to hydrate and fuel your ride.
Find out what works for you on say a 100 KM tide. This will work on an Imperial century as well.
Finally, this rarely gets mentioned, design a route that will captivate your attention so that the ride does not become a chore but an adventure.
Great scenery makes the miles fly by.
Good Luck!