Do up a cue sheet and map for the route so that riders can find their own way there, and indicate that they can ride at their own pace. Don't require riders to stay together in a bunch ... that's just asking for an accident.

Incidentally ... 10.5 miles to a park and 10.5 miles back is not an epic ride. It's a very short ride that would not likely appeal to "A" riders. The type of event you're talking about would most likely appeal only to very casual recreational cyclists. I doubt you'd have a problem with fast riders showing up to something like that.