I've been slowly getting better with my bash skills by organizing it as a class on my learning website: http://curiousreef.com/class/bash-scripting/

It's useful because it helps me stay on track. I can take off a few days or weeks even and then go back and do a few bash problems out of that book. I know that I'm 38% of the way through it, and it's easy to pick up where I left off.

I don't think I've done the peripheral learning that you mention, but I think 30 minutes a week is still low-impact learning.