On the Seagull Century, even though it has about 8,000 riders, it is still fairly easy to avoid pacelines. Start as late as you can, since the people who are trying to do a 5-hour century tend to start earlier. As pace lines come up, they will naturally pull over to the left to pass you. As they go by, avoid accelerating to hop on their tail. There, you're not in a paceline!

Most of the injuries that I've seen on the Seagull have been paceline-related. So I'd recommend avoiding pacelines. Just ride your own ride at your own pace, and all will be fine since that is how you've been training.