I recently ran into this on a laptop I was working on.
The laptop was extremely under-powered. Slow processor, minimal memory, etc. So I suspected that, that might be a contributing factor and decided to let the process fun for hours.
No luck, the download seemed to take forever would often just collapse and restart the checking for updates process over again from the beginning, which also seemed to take forever just to find it again.
I gave up and went here
https://catalog.update.microsoft.com/v7/site/Search.aspx?q=KB4034674
You'll need to use either IE or maybe Edge for that site to work.
AND you will need to get the appropriate version for your system (32 or 64 bit) and avoid the Delta version and just get the full version in the list.
BUT even then the process was taking forever when running the stand-alone installer.
So after exhausting my patience, I ended up manually stopping the appropriate services and renaming the Software Distribution folder as well as the Catroot2 folder, rebooted and ran the stand-alone installer and everything completed as expected.
While I worked it out manually there is a powershell scripted on Microsoft TechNet that looks like it will take care of the reset steps for you
https://gallery.technet.microsoft.com/Reset-WindowsUpdateps1-e0c5eb78#content
