There are two types of centuries that I am aware of. One in the US is usually just referred to as a century (100 miles) and the other is the metric century (100 Kilometers, or about 62 miles) I have heard the ride of 100 miles refered to as am imperial century, but never English, so I am not really sure about that.
Look at some entries for centuries here and elsewhere, search the internet.
I don't know if I have talked about it in this forum, perhaps it was at bikeforums.net, but I rode a metric century in October 2004, and I am a big encourager of riding centuries and other organized rides.
I am in the middle picture on this page:
http://www.photocrazy.com/perl/view.pl?ev=374&sd=2004-10-10&st=12:31:44&lfn=P041010123144541dk.jpg&d=b&nts=0301&bibno=
When I rode, I was on my stock Giant Sedona DX (comfort bike leaning toward mountain) with high pressure slicks. I had a rack, and was carrying more stuff than I should have, and was running thorn resistant tubes... so I was on a bike as poorly suited as any other on the road that day.
I also weighed just over 300 pounds and I was 47 years old... I was probably in better shape than any other 47 year old 300 pound man, but I still had those things working against me.
I chatted with some people on the ride, and was encouraged by many people, and given strange looks by some others.
At the end I could barely get off my bike, and once I did, I went in and had a wonderful meal and then went out to my car to leave. I sat in my car and cried tears of joy for a few minutes because it struck me what I had accomplished that day.
It was the best day of my cycling life.
All I can say is DO IT! Before that ride, the longest ride I had done in the past 30 years was 40 miles... That day, because of a wrong turn when I started out (I misread the cue sheet because I was in a different parking lot and started out on the 100 mile route) I rode 68 miles.
Remember that on an organized century you will get suport from other riders and sag wagons... supplied rest stops, and the ride of your life.
As you ride along and chat with various people let them know it is your first century. You will be shocked at the level of support... and you will find some in the same boat
And remember, the sag vehicles are there in case you can't make it. And there is no shame in using them... After all, you did at least try!!!
Neither century is easy the first time (I am making an assumption that 100 miles is harder than 62), but I don't think they're supposed to be.
General guidance for during the ride: Ride slow and steady, take the breaks, eat, drink, encourage and be encouraged.