Found it on sci-fi writer Jamie Rubin's fantastic blog, buried about halfway down his post on "How I Work", under "My Best Time-Saving Trick":
"Practical multitasking. I combine exercise and reading into useful mental breaks throughout the day. About three times each day, I leave the building to walk. At 10 am and 3 pm, I walk a brisk 20 minutes or so. At noon, I spent my entire lunch hour walking. While I walk, I listen to audio books. This allows me to get in exercise (the only exercise I get, really) amount to about 100 minutes of “very active” walking each day. It comes out to about 7 miles/day. At the same time, it allows me to read far more books that I have ever been able to do in the past. And it gives me a good mental break from work so that I come back to my desk refreshed."
Already in the habit of taking walk breaks, I found the idea of adding audio books (and, for me, Pimsleur French courses) a great way to forget about mentally tracking (and reviling) how many steps or miles or minutes you're walking and actually get through more of them. Happily.
Well played, Monsieur Rubin. Well played.