Biker395 

Bottom line: If you can ride X miles in reasonable comfort, you can ride 2X miles in reasonable discomfort.

If your goal is to ride the 100 miles like it was a walk in the park, you need to train by going longer distances. But if your goal is to finish and you're willing to deal with reasonable discomfort at the end, you're probably already there.

As for the century itself, I'd say:

Eat early and often. You might lose your appetite as the day goes on.
Drink early and often. If it gets warm, take/drink something to replenish your electrolytes.

One other thought:

Even though riding 100 miles is your goal, don't ride it looking at the odometer the whole time, counting off the miles until the end. That will make it seem to take forever. Just get on the bike and ride. Enjoy the scenery. Chat with other riders. And before you know it, you'll be done.

I think it's human nature to be goal oriented. And at the end of a long ride, you find yourself counting off the miles as if there is some kind of goal to be achieved. Mentally, that negatively affects your enjoyment.

One day, I got to thinking: I love cycling. I think about it when I should be thinking about other things (e.g. work). I sit around and look at pictures of my friends and I on our cycling adventures. Given all that WHY am I ticking off the last few miles as if I'm glad to be done with it?

So instead, I try to focus on being in the moment and enjoying what I'm doing.