How do you get from GNR to the Washington Monument in Fallout3?

Kay, I'm doing the quest to try and find dad, and I've spoken to Three Dog at GNR. He's asked me to go and fix his radio antennae at the Washington Monument building. I wandered around for a while trying to find it, and got lost. I made it to Dukov's Place, then walked all the way down to Rivet City, discovering it accidentally. How the HELL do you get to the Washington Monument? Please be specific, and detailed, use location names not just "Oh, the metro station". Do you know how many metro stations there are? Yeah, lots. Thanks.