Books and courses are nice and helpful, but the only way to really get in and do things right even as a junior sysadmin is to get a job as a SysAdmin/SRE/DevOps/Network Engineer so you can work with others with real world experience. Without it there is absolutely no way to get to the level of expertise needed to get to the top of the game. As with the really important stuff, there is always a team making it happen.

By getting a job you will experience the good old production environment which is something books and courses cannot teach you, especially when you have to deal with different types of environments/people that only SysAdmins, Network Engineers, DevOps have the responsibility to manage and secure and keep up (24/7/365). As most mission critical environments are extremely fast paced and very sensitive to accuracy and due diligence you will learn some serious skill sets that you will get from working in those types of environments.

If you want to learn quick and master a large spectrum not touched in the books working on a mission critical team will get you up to speed especially if it is for the government (NASA, Military, NSA, CIA, NGA, SCC, NRO, FEMA, Secret Service, FBI) or a not so critical but high profile and important commercial company (Google, Apple, Facebook, Uber, Twitch, Amazon, any top defense contractor, etc.)