Relax. Have fun. Go at your own pace. You will be proud of yourself after you finish! I was worried about my snail pace in my first century, but there were more cars in the parking lot when I finished than when I started 
(Check the SAG cutoff--our end of the year century has a 12 hour cutoff for support--otherwise, have a backup plan--a friend that can rescue you, if you need it)
I also recommend Cytomax instead of Powerade on longer rides. I'm susceptible to bonking, if I don't get in enough calories. I also snack on jelly beans, cookies, etc while riding--on top of the bars on my stops.
"Well-behaved women seldom make history." --Laurel Thatcher Ulrich
'09 Trek WSD 2.1 with a Brooks B-68 saddle
'11 Trek WSD Madone 5.2 with Brooks B-17