At this late hour (near 2AM here, and long after your post) you still may
not have the download completed. IF you are using wi-fi connection
and not an Ethernet connection, the 'download speed' may be glacial.
A wired Ethernet connection direct from the download Mac App link
could be many times faster; so instead of taking up to two days, it
may only be six or eight hours.
The factors to consider include the bandwidth your ISP allows you
for the monthly fee, the ceiling on the total you can use in a month,
and if they shut down the speed to a mere trickle after a while, if
you have already used most of the allotted capacity available to you.
And that final link to the computer, be it wi-fi (slower) or Ethernet (best.)
Hopefully by now you've had some headway on a download. Also if you
choose to not downloaded installer, you can save it to the Applications
Folder by quitting the installer. You may want to read up more before you
install the El Capitan 10.11.6 version. And backup the old system, too.
Good luck in this matter! 🙂