Take a look at the specific date and you may find a three-class aircraft, typically a 777 from IAD, rather than DCA. Book that in C. It's much nicer than F, especially nicer than F on an E-70. Three-class is simply when UA needs to move aircraft around, so don't rely on it for any particular day/time.

Also, take a look at A fares out of DCA, BWI and IAD. It may well be that you can purchase a seat in F (or C on 3-class) for a minimal amount more than Y, especially if you can live with a non-refundable.