I play with Linux now and then, nothing serious. You may want to check out Oracle Linux, version 6 for 32 bit machines, version 7 for 64 bits. I installed the 32 bit version native on an old 32 bit laptop and use the 'YUM' updater once in a while to keep it up to date. There's a GUI desktop if you want to use that and you can always open a terminal window for command line. The downloads are free from Oracle, and these are Enterprise class products. If you don't want a native install they work under Virtualbox as well.
A smaller alternative you can carry on a USB drive is Tails. TOR is built in, there's a GUI and terminal window interface. It comes with a number of handy apps like video and audio players, Libre Office, and a few others, but some commands are not available if you're trying to learn. It installs on an 8GB or larger thumbdrive. I keep an install of it on a partitioned thumbdrive on my keychain. It has built in encryption and will open Truecrypt and Veracrypt files. I find Tails pretty handy to have around. Another nice feature is it doesn't even need a hard drive to run, it's all done in RAM and leaves no trace of what you were doing behind.
Solaris may be of interest. It is an Enterprise class product, a real workhorse run on many Oracle servers. We have servers running it 7x24 for a year or more without a glitch in heavy commercial use. SOmetimes we are amazed at how long it can stay up and running. There's an x86 version that can be run on Virtualbox as well. Downloads are free from Oracle. Version 10 or 11 would be your best bet.
Of the above, Tails is what I use most to do stuff. For learning, some important things like user accounts are not available. The Oracle software is the other extreme, true Enterprise class. If you really want to dig deep into Linux the Oracle stuff may be worth looking into- and I am sure there are others. All are free downloads, and Oracle has lots of documentation. I really haven't tried other flavors of Linux. I work for Oracle, hence all the Oracle ideas. Hope that's of some help.