Just to point our two major differences, there is Linux desktop use and Linux server use. A lot of desktop specific stuff you will have not have much use for when being a sysadmin, but basic stuff you use as a sysadmin you will also use on the desktop.
I can only tell you how I learned and that is by experience and trying out things. I dual-booted for a few months when I started, a short time after I took the leap of faith and wiped my Windows partition. I then tried to do the things I was used to doing on Windows, when I ran into something I would search the web. For example I did read a book to go through some basic commands and learning how to work with package managers or some website, can't remember which. I didn't know how to install video drivers so I would search how do I install Nvidia drivers under Linux, back then it wasn't as easy as it is now. Basically in short 90% is hands on experience and 10% reading searching the web and reading about how others did it. Set yourself a project, learning to use Linux as my desktop and try to solve the problem you run into.
Lastly it also depends on how much free time you have on your hands, at the time I started with Linux it was my hobby and I spend all my free time on it. How much you put in is how much you get out of something. I hope that helps, and lastly I have zero interest in Kali and pen-testing tools because Linux is fun enough for me since the learning never stops because of technology development is never sleeping and I have no interest in being tempted to do something illegal