Welcome to the best "major" airport in the Northeast -- close to downtown, very easy navigation, and this being "Congress's Airport" you can get virtually anywhere in the continental US directly. Your schedule should not be an issue; even at its worst it should not take more than 30 mins to get from 14th & F to DCA. (Use a Lyft / Uber instead of a cab; don't bother with Metro.) Typical TSA Pre wait time is under 10 minutes, and the walk to the United gates is less than five minutes from there.

If you need to eat, go toward Gates 20-22 straight ahead past security instead of turning left to the United gates; the options are much better. Also, the United Club is a bit tucked away, upstairs near Gate 11. It's a retro Club, not big but perfectly nice and quieter than most.