This is a link to a ride I do somewhat regularly and this ride description seems to get only "serious" riders. The Cascade Bicycle Club has a ride calendar where all kinds of rides are listed, so there's the benefit of uniformity when it comes to pace descriptions or amount of elevation. Might give you some ideas for wording though.

http://www.cascade.org/EandR/Activit...?eventID=13835

In the end though, I'd list a pace for the ride, describe who you want to come like "people wanting to learn to ride in a group", or "people experienced with riding in groups", List mileage, and then whether it's a "no drop" ride or not. That should let people know what they're getting into.