Organized rides are rarely group rides. People start at different times (of their choosing -- those doing a longer option tend to start earlier) and ride at their own pace. Hence, I would not have much angst over it. One key thing to remember is to ride predictably: keep your line and keep right, so faster riders can pass safely. Keeping your line is crucial during turns. Other than that, enjoy the ride. It is nice to have a route set up for you, with rest stops and probably sag support if a bike breaks down.