I'm not sure about your speed calculations - average adult walking speed is generally understood to be around 5-6 km/h (3-3.75 mph). I'm a male in my late 30s and I generally walk at a relatively brisk 7-8 km/h (4.3-5 mph).
As it happens, my walk home is about twice as long as my walk to work, since I pick up my son from school along the way. My walk to work takes about 20 minutes and the walk home takes around 45-50 (part of it at a child's brisk pace).
> This means the cost/benefit would go something like this:
You're missing some factors from your cost/benefit analysis.
* Walking to work means we only need 1 family car. That saves $10,000 a year in direct ownership costs, not including paying for parking.
* If I drove instead of walking, I would not use the time saved for exercise at a gym (know thyself), so I would simply miss that daily exercise. But if my commute just happens to constitute daily exercise, so be it.
* There are about a dozen restaurants within a 5-10 minute walk of my office, including some of the best places in the city. (However, I'm cheap and almost always pack a lunch anyway.)
* Downtown, not having a car confers more flexibility because I don't have to figure out where to park it.
* If I do need more transportation flexibility on a given day, I can always take my bike to work that day.