If you have not booked your ticket, look for a ticket to Washington National Airport (DCA). Even if it's a bit more expensive, you'll save in taxi costs.

You can take the subway (metro) from DCA to Union Station. One change. You can also take a taxi and it will be much cheaper than taking it from IAD.

I recommend hotels.com if you are looking for a good price. Use the map feature to find somewhere convenient.

Also a note that Amtrak trains usually leave on time from DC but if you are returning Martinsburg - DC, the train originates in Chicago, and may be later than advertised; something to keep in mind as you book a homeward flight.