Sorry, but having said that the transfer was doable, armed with the additional fact that OP is on separate tickets, I would now not do this. If one small thing goes wrong, OP misses his xIAD flight and he's not a connecting pax so he's simply a no show.

While UA, unlike some foreign carriers, will likely rebook OP, it's not obligated to do so and OP will come behind all other IRROPS pax. At a minimum, OP will be stuck with the costs of hotel + food and the wait for the next flight with availability.

This is one of those deals where OP will be fine if everything goes according to schedule, but will have a substantial delay and maybe substantial costs if things don't.