IMHO, you need to ride at least 2/3rds of the time you plan to spend in the saddle during your rides. You need to be at a pace that you can hold for the entire century, not the pace you can hold for the 2/3rds of the time to ride a century (or metric century). During these rides, you need to practice your fueling and hydration strategy.
If you can do this, you'll have a better chance at having a good experience, otherwise, you'll probably suffer a bit by the end of the first few attempts, until those rides cause an adaptation in you to handle the time in the saddles sans pre-century training.