Just want to give an update now that I am back. When I checked in for the first flight UA7988, the ANA agent assured me that people have connected in NRT in as little as 20 minutes, so I had nothing to worry about. UA7988 arrived NRT 45 minutes early so that gave me an extra 45 minutes. Without those extra minutes, I am not sure if I could have made it.
There was an agent at the gate as soon as the first flight landed. She had a list to use to check off my name on the connecting flight. After that, it was the walk upstairs to the security line. The security line was not too long and was relatively easy. It only forced me to ditch my water bottle and rebuy another one after the security check.
The walk to the next flight was surprisingly long with turn after turn in a crowded terminal. It must have taken me at least 15-20 minutes at a normal pace. By the time I reached the gate, they already started the boarding process, which means overhead space started to disappear.
If I have to do it over again, I would pick a longer connection time at NRT. There were attractive looking ramen choices along the walk that I had to force myself to pass.