My (unintentional) Command Line Challenge was trying to build Gentoo on a 100MHz laptop with no other computers around. That was about 2 days of compute work. At the time, it didn't compile reliably either, so emerge would crash 5-10 times while trying to get to Xorg. Learning to fix the problems, particularly via lynx, brought me up to speed more quickly than anything else.
Something to be said for leaning by immersion.