You might try Seneca Creek Park
off Clopper Road between Gaithersburg and Germantown. They have a nice single track trail system that includes beginner and some intermediate levels. The single track that loops around the lake is very scenic and easy enuf for a beginner.
Trails listed here: http://www.dnr.state.md.us/publiclands/senecaguide.html
Word of advice learned the hard way: park outside the gates! When the gates close at dusk any cars inside are pretty much there for the night. The trails are short enough to make a nice, after work, evening ride, during the summer.
Other good choices for newbies are the mid and upper reaches of the C&O Canal
. Just jump in at any point above the Great Falls; just about anything north of the mouth of the Monocacy is pretty country: American history the easy way!
Best site for all things C&O: http://www.mcmullans.org/canal/
... pick a section and enjoy!
There are a lot of places around the DC area but these two are probably the most interesting and the easiest to get to. We always start our biking season with a Sunday ride on the C&O, about 12-15 miles out and back, followed by a big breakfast at our favorite deli. The way the weather is behaving that ride might be sooner rather than later .. but ya never know ... historic snow dumps are not out of the question even in late March around here.
Just thinking out loud .. but would there be any interest in a group ride of DCSki folks? C&O is dirt trails above the Falls, so skinny tire folks might not have much fun.
Also ... anyone here into kayaking? The touring kind, not the 'oh my gawd, I'm gonna die' white water kind. We are looking for interested bodies to share the zen of bow waves, bald eagles and bugs. Takers?