I would get to know the bike path really well - I think that's the one I rode on visiting my brother in Reston. Get comfortable with what you're supposed to do when (where it crosses roads, etc) and maybe use the clock as your guide... go *out* for fifteen minutes and then turn around and go back.
And, don't feel intimidated by more expert riders going by you. You should try to be sure to ride a straight, consistent path (and when you pass somebody, let 'em know with a "hello!" or "On your left!"). Know what's going on in front of and behidn you (I swear by my helmet mirror) and figure that people will think at first, "Newbie! Wonder how quickly she'll give up on that!" and PROVE THEM WRONG so that in a few weeks they're thinking "Oh, it's that lady! Boy, has she gotten stronger and more confident!"
Oh, and log your miles or minutes, even the half miles and half minutes :-) Stick 'em on a wall calendar, or on a computer file, or over at www.bikejournal.com - but there's nothing like visible progress to motivate.