Sorry to say, there's no such thing as a Domestic First Lounge at UA. There are three Red Carpet Clubs, but you will need to be a member or pay a $50 one-time entrance fee to get in. I would also try and stay gateside to avoid the security lines which are bad enough for elites and beyond bad for non-elites.

Given your itinerary, I'd just as soon fly into DEN early and hang out there; better restaurants, comfortable waiting areas, and free wi-fi are nice. It's probably not an option for you with your upgrade, but if I did find an earlier flight with space in F, I'd probably go to DEN early vs. hanging around IAD.