Rivnuts from Homestead, PA on 2/17/2020 8:43:00 PM:
Union Station is about 4 miles from The C&O Towpath Milepost 0 at the Thompson Boat Center and 5 miles from Reagan International Airport which would be another potential rental car drop off location. You can ride the Mt. Vernon Trail for much of the way from the airport to Milepost 0. There is also an Enterprise Car Rental location on Calvert Steet in the Georgetown section of DC and is about 2.5 miles from Milepost 0.
If you are spending the first night in Potomac MD, it would seem that you would have plenty of time to take the train to Washington's Union Station which arrives from Pgh around 1:05 p.m. and get to Potomac MD. It's less than 20 miles from Union Station to Potomac...so, say, about 2 hours. Depending on what time of year you are going you could make it to White's Ferry/Leesburg (40 miles) before dark.