OP - Although the weather is good today, check flight status carefully. If your flight is substantially delayed or cancelled, do not head out to DCA from the NPC, but head straight to Union Station (5-10 mins by taxi/uber/bus/metro, 20 min. walk) and grab a train. I presume that you are connecting at EWR, so you will need to call UA to have them cancel your DCA-EWR segment so that your onward/return segment(s) don't get cancelled, but you are entitled to that in case of delay/cancellation.

For what it is worth, if you leave the NPC in an Uber at 5:05 (you can now preset the pickup time) and give the address location for the building on 14th Street facing South, it will take 15-20 minutes to DCA at that time of day. With Pre-Check, you will be at your UA gate in 10 minutes (only because the EWR departures are down a long corridor from the checkpoint). Without Pre-Check, add 10-15 minutes for the checkpoint at that time of day (even w. status, because it's DCA and everybody has that).

Amtrak is actually a far easier and better option than the miserable micro-hop up to EWR which is routinely delayed/cancelled because late afternoon ATC results in those flights getting the short end of the stick.

If you do wind up on Amtrak, don't short-change yourself on comfort. It is substantially nicer to take an Acela changing to almost anything including a local commuter on NJ Transit at Metro Park than to take something less pleasant from WAS just because it stops at EWR (which the Acela does not).