It should not take that long. I have had upgrades hang. I have closed it and
then run sudo apt-get -f install to complete it. If you get an error that
the package manager is locked then you will need to log out and back in or
even re-boot. You can remove the lock manually by navigating to its
location, too, as root. These things are usually just temporary
inconveniences.

It eventually finished. After I sent this, I got busy with something else. A few
minutes later, I looked back, expecting it to be at about 15%, given the rate at
which it had been going. Instead, update manager was informing me that my system
was up-to-date. Don't know why it was running so slow for a while or why it
picked up and finished so quick, but it did.