Seriously, you are going to do just fine if you find a good pace and just keep it. Make sure to drink, eat and rest before you think you need to. If you wait until you are actually feeling thirsty or hungry or tired, it takes you a lot longer to recover and you'll be heading straight for BONK. Try to enjoy the event, like Ariel said, it isn't a race and you will feel a lot better if you finish strong than if you trudge through the finish line with your last bit of energy. Since this is a big group ride, it should be fairly easy to find a nice group that is going your pace.

My GF just finished her first century and she had major doubts about whether she had trained enough. I kept telling her she was way more prepared than she thought. She finished and felt great at the end.