55 minutes Int. – Dom. connection is very tight, I will not feel totally comfortable. Bit surprised that you got ticketed this way when NH recommend 75 minutes or more for Int. – Dom. connection at NRT. I understand that ticket was issued by UA…
I think good point is that you are transferring from NH to NH under one ticket. NH will be aware of your connection, ground agents at NRT will likely be aware of your connection at NRT. There should be NH agent at the jet bridge as you deplane at NRT. Mention to an agent of your tight connection, I am sure NH agents will do their best, often radio in hand communicating with other sections and running to a gate with a passenger.