I need to think about this some more, but off the top of my head:

In addition to the Potomac Pedalers, there's also the Oxon Hill bike club: www.ohbike.org. I've never actually been on their club rides but I know at least one person here used to ride with them (Tulip, maybe?).

Also, it sounds like it might be good to have a goal for you to work towards. The OH club sponsors the Patuxent River Rural Legacy ride every year in early June. This year's ride is on Sunday, June 12. It's one of my favorite rides every year. You can choose between 24, 46 or 62 miles. You might want to aim for the 46 miler, and you could gradually increase your miles from 30 up to 45 or so between now and June.

The same club is co-sponsoring a ride on April 24 to raise money to fight hunger in Calvert County, MD. (The fundraising is just your registration fee; there's no need to ask friends for donations or anything like that.) I did the 36 mile version last year and this year I will either do that again or do the 48-miler, depending on how much training I'm able to do in the next month or so. It's a nice ride that goes along the western shore of the Chesapeake.

Both of these rides are in eastern MD, but on this side of the Bay so the drive to the start isn't too far.

The good thing about these kinds of rides is that there are other riders around, reststops with food, drink and bathrooms or porta-johns, and SAG support if you have a problem. And they get you off your regular weekend route and onto some new roads, which is always nice.

There are other organized rides like this in Maryland and NoVa. Here's a partial list which includes the Patuxent River ride and the Ride to End Hunger in Calvert County:

http://ohbike.org/events/index.htm