12 miles is IMO the best commute distance, Over my working years, I've had commutes from 2 miles to 17. 17 and a storm headwind is too hard! 12? I've had 4 jobs with that distance. I can build to riding every day, it's doable in just about all weathers. Not too draining. Doesn't take too much time.

One way to pack a bigger bang into your fixed commute miles is to ride it on a fix gear. (If you commute year 'round, this has the added advantage of making the downhills a lot warmer!) A fix gear is roughly 25-33% harder than the same distance on gears. Does considerably more for the rest of your body if the commute has any hills and you don't have time to lift weights.