The canal is great because you don't have to deal with cars at all and on weekdays there won't be too much foot traffic/kids/dogs, etc. The fastest access from Rockville would be I-270 south toward Northern Virginia, where it will merge with the beltway (495). Get over to the right (you'll have plenty of time) and take the Cabin John Parkway toward Washington/Glen Echo. That will merge onto the Clara Barton Parkway heading toward DC, and there are various pull-offs/parking lots where you can park right next to the canal. Go downstream (left facing the Potomac River) to get to Georgetown, or upstream (right facing river) to go toward Great Falls.
An alternative is the Rock Creek trail which passes just east of Rockville along Rock Creek. This trail goes all the way to Georgetown and can be accessed at dozens of places. Find Rock Creek Park on any local map or mapquest for roads that cross or run along the park. If you ride all the way to Georgetown, keep in mind that you will be heading upstream on the way back, and, depending on your condition, this can be a factor. (I prefer a steep hill rather than a long, slight incline).