As a commuter I did this all the time. I found I could actually do the trip quicker on the metro because the bus still can be affected by traffic. From IAD get on the Silver Line Express Bus. $3 for employees have to buy a ticket at the desk. That drops you off at Whiele-Reston East. Take Silver Line to Rosslyn, then Blue Line to DCA. Same route in reverse but when you get on the Silver Line Express going to IAD, the driver will tell you to just buy a ticket once you get to IAD. If timing is good I’ve done it in as little as 1:10. Just be aware that the Metro doesn’t open very early or stay open late so make sure to check times. Download the DC Metro app and it will give you Travel Alerts if there are delays or track maintenance.