I read this article where a leading environmentalist suggests walking to the shops can actually damage the environment more than if you were to use your car. His calculations are based on the fact that with food production being so energy intensive more carbon is emitted providing a person with enough calories to walk to the shop than if you were to take a car over the same distance. I find this article a little silly. People need to eat whether they walk to the shop or not. I eat the same amount every day regardless of whether I walk or do other exercise. If I were to require more food for my walk to the shop, perhaps he might have a case. Either way, I will continue to walk and leave the car at home.
