On the Sat. rides if you plan to do the whole ride but know you will not be keeping up with the main group you should inform the ride leader before the start not to worry if you drop back. Also take a good look at the map/cuesheet and ask questions before the ride about parts of the route that are unclear. When I lead rides and folks tell me that then I know it's ok to go on. Before the start you might also ask others riders what their plans are and speed they plan to ride. You might find one or two that will partner up with you.
The same I would say goes for Tuesday rides. Just inform the ride leader your intentions and again ask other riders if anyone is looking for extra miles and at what pace.
Hope this helps, Cary

Yeah, what Cary said. Don't let your speed keep you from riding with the club. There are people of all speeds in the club. Also, the more distance riding you do, the faster you will become.