As most seemed to have recommended, building something or starting a long project and maintaining it are the best ways.
If you want some free resources, I would suggest:
If you want some books:
Other paid resources:
I’m sure some other people can suggest some great, or better resources. But those have helped me gain, and reinforce existing knowledge. Really top tier stuff. I would also like to recommend Linux Administration: A Beginners Guide. Some people have criticized the book as being too hardcore or biased towards Linux. Sure, but they’re not as obnoxious as the people are on Reddit or anything like that.
I would recommend building a couple of web servers with different architectures. You don’t have to know how to code, just need to know how to follow instructions. NodeJS, Apache, Nginx, and Python would be good to springboard into.
I’m going to introduce my personal beliefs, which have been known to upset and offend more than a couple of people. Please, don’t be upset, and don’t be offended 
I do not recommend Arch Linux. I do not recommend Slackware, Gentoo, or Linux From Scratch. Not yet. As a hobbyist, sure, go for it and cut loose. But if you are looking to be productive on Linux, I recommend picking Debian, Ubuntu, or Fedora and sticking with that. The Arch wiki and Gentoo wiki pages are useful at times, but they are not the be all end all. In my experience, gaining useful, productive knowledge with Linux, especially with first time users or novices, is best served in a well maintained and well used distribution.
I am not a pinnacle in the Linux world, or even in the open-source world. I’ve taught a handful of people and I’ve used Linux professionally for a few years. Take what I, or anyone else said, how you will. You will benefit greatly from something if it is practical. Build a Nextcloud or Owncloud instance so you can access your files from anywhere. Create a security lab with KVM, and use Linux distros as routers to secure your lab. Build a website to host your resume and digital portfolio. Build a wiki for storing knowledge you’ve gained as you learn more about Linux.
All of the above will go along way then say learning DNS Bind or NIC Teaming. While those are useful, start with something practical, make that your goal, and strive to make it happen. You will be surprised with what you learn and what you can do, especially with so little.
I would also like to add that how you use Linux doesn’t matter. There is some elitist Internet mentality that if you don’t install Linux baremetal and use a 16 bit tiling window manager that you’re not a real Linux user. Do not let these people intimidate or bully you. If you’re ssh’ing into a Linux server from Windows or using a desktop in VirtualBox or VMware Workstation, you are using a Linux system.
L1T is a great community, so if you have questions be sure to post them. I’m sure myself as well as others will be more than willing to help.