Are you sure you want to take route 29? I live on 29, work on 29 and drive 29 29 times a day.
But when I bike I try to STAY OFF 29.
Are you looking for the direct, heavy traffic way, or will a more scenic route work?
Roads only or are trails ok?
To Baltimore - if you can get to York, there is a rail trail
http://www.waba.org/new/paths/northcentral.php (crushed stone but an easy ride) that goes 45 miles to Timmonium, MD, which is almost Baltimore. You can get to downtown Baltimore on the York Road or cut west to North Charles and Saint Paul Streets. Maybe one of the Baltimore locals has a suggestion on where to go from there. There are supposed to be some routes through Anne Arundel County to Bowie. From there the Washington, Baltimore and Annapolis Trail gets you almost to DC
http://www.waba.org/new/paths/wbamapframe.htm The map has some other bike routes into DC. One possible set of trails is
http://www.waba.org/new/paths/atsmapframe.htm
You could also skip Baltimore and come down through backroads in Delaware and Eastern Shore Maryland to the Kent Island Bridge and Annapolis/Anne Arundel County.
Or go west to Gettysburg and come down the roads parallel to route 15 to Frederick, then head west to the Potomac River and either cross to Leesburg or go south on the C&O.
Once you get near DC, take any of several trails downtown (recommend the Capital Crescent) go to Georgetown, cross the Key Bridge and take the Custis Trail west (the ugly highway on your left before you get to I-66 will be route 29). Trail T's at the WO&D. Turn right and go 40 miles to Purcellsville. From there take the good country roads south through Philomont, Middleburg, to The Plains. I'm less familiar with the bikeable roads south of there, but you should be able to parallel route 17 and route 29 the rest of the way to Charlottesville. At that point route 29 itself might not be too bad.
a couple of web sites with cue sheets or maps.
http://ohbike.org/
http://www.bikepptc.org/
http://www.waba.org/new/paths/index.php
Good luck. Have fun. Wave to the left side in Vienna if you go down the WO&D.