There's also Oxon Hill Cycling Club (ohbike.org). Many of their rides are in Maryland, while PPTC are in Virginia mostly. Lots of overlap in the groups.

Keep riding with the Ds until you feel you can do one of the shorter C rides. The C rides consist of a variety of speeds and almost always have a sweeper who makes sure that folks don't get dropped. It's a good idea to have a bike computer and a cue-clip holder for cue sheets.

As Bleeker says, you'll soon realize that you can easily do 20 mile rides, and from there, 30 milers. A few 30 milers and you can easily do 40. Then they get longer, but you'll be doing those, too. Once you get comfortable in traffic, you might want to commute to work on your bike. It's a great way to build up miles and consistency, as well as a great way to get to work.

For learning to ride in traffic, Madscot's offer to go with you is very generous. There are occassionally cycling classes offered by the League of American Bicyclists and WABA (Madscot gives the web address).