Time of day will be critical for this commute. Assuming that you are taking the Circulator/MARC option, it will be particularly helpful to get out of Georgetown before 7:30. Any later and your commute time to Union Station will be both significantly longer and much more unpredictable.

The return is even more subject to the vagaries of traffic. Traffic from Union Station to Georgetown in evening rush hour is often horrendous. This is when finding a decent metro option - if you can walk from Foggy Bottom, Rosslyn, or Dupont Circle, it will really help.