I think that would be a fine ride for your tandem mountain bike. Most people would get out of the way for you and if they didn't you would probably fare better than them. We go on a ride once a year with a lot of others, but not as many as yours, and I have a hard time staying comfortable on my road bike for 4 hours at 10-12 miles per hour. We ride with my wifes sister, and that is as fast as she goes. We still pass people at that speed. What scares me on rides like these is that there is always a few groups riding in pace lines at speed. On our last big "event" ride I saw three crashes.