Originally Posted by
FliesWay2Much
There is a way to get to the north entrance. If you enter the trail as others have suggested (near the Water Park), you'll cross under the GW parkway.
Here's a little map that I drew of the direct-but-dangerous vs. indirect-but-grade-separated paths from the north route to DCA:
http://www.flickr.com/photos/paytonc/6266966394/
I sometimes use Capital Bikeshare to get to Crystal City and then walk the rest of the way. Haven't used the south route before, but since there's a CaBi dock right at the volleyball courts, it might be more efficient for UA flights out of Terminal B.