Walkability is a current planning trend that has implications for public health, traffic, land use planning, zoning, parks and pretty much everything we like about cities. All else equal, cities should favor more walkability over less walkability. Too often planners and urbanists narrowly define walkability as the ability to walk to places where they like to spend money*, and we should think more broadly about what walkable means for all types of folks, but in most cases more walking is better than less walking.
Say you live in a walkable community where you can do most of your errands on foot. How much might you walk on any given day? How much should you walk? Paul Krugman weighed in on this question on his blog. He notes that he walks about 15,000 steps per day, and this is "fairly easy to do." Since most people take about 2,000 steps per mile, he's covering about 7.5 miles daily. This is hard to do!
I posit that if you are walking 7.5 miles daily to go about your business you are not in a walkable community. Mabe farmers and ranchers cover that kind of ground, but at that distance your daily activities are too spread out for easy walking. That distance covered daily is getting close to the hunter-gather period where humans walked 3,000-4,000 miles annually.
An realistic pace for walking is about three miles per hour. This means that 7.5 miles is about 2.5 hours of travel per day, not including subway, bus, taxi, etc. It is certainly doable, but not simply by going about your business. You have to want to do it, and be able to afford to do it. Walking is a leisure good if your income is high enough. You can buy more enjoyable and slower travel and work a bit less. Most people don't have this option, however.
But how much walking is the right amount for a walkable community? I'm not sure, but one place to start is with established travel time budgets of around 90 minutes of total travel per day. This suggests walking about 4.5 miles daily, which is less than the doctor recommended 10,000 steps. I'm not sure this is right or wrong (it is probably too high as carrying things while walking or traveling with children limits how far and how fast you can go), but a good place to start when evaluating walkable communities. People should be able to take care of their daily business by walking less than five miles per day. If you use transit and live and work within 1/2 mile of your stops, then your commute includes two miles of walking. As the average person takes less than four trips per day total (Table 3 in this summary of NHTS data, and I realize that national data may not accurately reflect trip totals for denser areas) it seems unlikely that a couple more trips will lead to a few more miles, unless your destination are really far apart.
In sum, to make a community walkable it must have lots of things close together. The closer together these things are, the fewer miles traveled it takes to get to them. So the most walkable places are those where people can access the most while walking the least. If you are walking 2.5 miles per day for all of your activities--which is about what I average about forty blocks north of where Paul Krugman lives (somewhere on the Upper West Side as I understand)--then walking places is easy.
Since each mile is associated with about 100 calories burned, we should also temper our expectations that the marginal increase in walking will offset the increase in calories recently seen. Five miles daily is not enough to offset fancy coffee drinks, sports drinks or even eating out a couple of times per week.
So walking is good and we should encourage more of it through design, mixed uses and street redesign. We should also keep in mind that too much walking suggests that our communities are not actually that walkable and people walk a lot because they can. To be truly walkable people should be able to reach all of their destinations without walking far. Ultimately, however, if we really want people to walk more the best thing to do is get everybody a dog.
*Central Park is one of the best places in Manhattan to walk, but for the most part you just walk about and enjoy your company, the trees, turtles, an occasional bird, etc. Central Park is not a great place to walk because you want to have lunch at the Boathouse. Fancy people take a shuttle there.