Walking is a great time to listen. Also when doing (mostly) mindless work, like washing up or cooking familiar meals or watering the garden. As for speed, that's controllable -- I find that I sometimes like to listen at real speed, other times up to 2x or occasionally faster, often somewhere in between. (This doesn't just depend on the speed of the original, but also its information density, how important the social/emotional human element is, what I'm doing, and what mood I'm in.) Faster speeds can actually aid concentration, because they give your mind less reason to drift away whenever there's a pause or a predictable sentence or a dull bit. This all goes for audiobooks, too.