I've been using Premier Pro a while and I've had some problems here and there when it comes to exporting my videos with it or Media Encoder, but I can't remember the last time it took me 20 hours to export a video that's under 20 minutes long.
I haven't changed my export settings from when they used to be reasonable export lengths. The only thing different that happened was I had to use Handbrake to turn my video from variable to constant framerate and I had to argue with it to turn from a m4v file to an mp4. The file says it's an mp4 now but it still acts like an m4v file. Does that have anything to do with it?
I just did a test and it's exporting for about an hour now.
For anyone else with a similar problem, all I did was uncheck "Export with maximum render settings". I was told once it wasn't necessary, and though I don't remember saving it with my exporting preset, it seems to have done the trick.