Normally, United arrives between C2-6 and this fall U.S. Airways arrived and departed from C 20-23 area. You have to go all the way down C to the Main ticketing level, go back up the to the skybridge get to the hotel, make a left and go to the back corner to the reception area to do the paperwork, then back to the elevators, up to your room, however long it takes to freshen up, then reverse routing to get back, now going to security, again. Granted, you will be going back out on a domestic flight so you will not have to clear customs three times, but you end up getting tight on time, here's where you would lose. I would budget a good 2.5 - 3 hours. Your best bet, because you have an Envoy ticket is to go the the United Club (the Senator has a similar set-up around the corner) and check in there, talk to the security lady and ask to sign up for a shower. I goes in cycles of how quickly they get people through, but you are in the club, getting updated flight information and enjoying free booze (Wow, I don't think I've made a post without mentioning alcohol). There is finger food there, but nothing substantial. The shower booth is adequate, they supply the towel, toiletries and rob there is enough room to retrieve a change of clothes and any speciality items you might need. If you still have time to burn, you get grab a bite at one of the shops in the publik areas. Otherwise you can go back down the escalator and pop into the duty-free area or there is a newstand near C-2 that usually has sandwiches.
They should be done re-remodelling the terminal and there are quiet a few shops to check out to kill time if need be.