The math is silly and based on wrong information. Maybe painting with a broad brush, but ever since Freakonomics, it seems economists think they can turn everything on its head by applying some math. Not realizing that the assumptions/inputs are more important than the formulas.
-Hementions the energy costs associated with food production (he claims 15-20 calories cost per calorie of food produced - a VERY high estimate). He attributes food that is thrown away or wasted to the energy expenditure of food production (i.e. adds that to the pollution associate with walking... which is just plain silly).
-He estimates walking a mile at 1-2mph burns 200 Calories above basal metabolic rate. That is about DOUBLE what any other resource will tell you.
So by artificially doubling the energy cost of food production and doubling the calories burned by walking, he's already quadrupled the energy cost/pollution of walking before even doing any math.
While looking at the energy costs of getting food to your table, he apparently omits the fact that extracting petroleum, refining it, transporting it, and getting your car to the gas station all burn fuel.
So, all the assumptions are essentially BS.
Doing a direct comparison of food oils vs gasoline:
A gallon of gasoline contains 31,000 kilocalories.
If you drive 1 mile at (an extremely efficient) 40mpg you are burning 0.025 gallons of gasoline (6.4 tablespoons), or 775 kilocalories of gasoline.
A 200 pound person walking one miles at 3mph burns about 110 kilocalories (less than one tablespoon of olive oil - for comparison).
Finally as others have mentioned there are all of the positive externalities of walking, and all of the negative externalities of car driving - do you add the wars in the Middle East, road construction, etc all to the driving side of the equation???