Most of the other comments suggest taking Metro. I'd suggest Uber unless you're focused on saving cash. It's, what, $10 to Metro each way? By contrast, unless there's a surge, it'll be about $50 with Uber (plus tip). So, much more expensive, but it depends what $50 extra is worth to you.

The benefit of Uber is that you get in the car, do whatever, and you're there. Traffic may be slow, but half of the ride is on the express road for Dulles, which won't have traffic. Worst possible case? 90 minutes. Realistic bad case? 50-60 minutes. Best case: 35 minutes.

Metro is not hard, but it's possible to get confused at the connection in Rosslyn. If you care about the money, Metro makes lots of sense. But if you're lazy like me, Uber is easier and will take you right up to the terminal.

On the return, you have five hours. You could accidentally board a freight train to Atlanta and still make your connection.