According to many estimates the average person gets the equivalent of 75 miles per gallon when walking. Compared to the average 24.6 miles per gallon of cars in the U.S. and the fact that walking is a zero emissions mode of transportation**, walking seems a pretty good environmental deal, and it is, but there are other sustainability related benefits to leaving the car parked.
Getting to know your community. - Walking allows you to see your neighborhood up close and personal in a way that you simply can't do when passing through at 15 - 50 miles per hour. You can get to know local plants and wildlife (yes, even in the city there is wildlife), frequent local business and say hello to your neighbors.
Health Benefits - Walking is good for you. I'm pretty lazy about exercise. I'm not the type to go to the gym or take an aerobics class. With my feet as my transportation I usually walk at least a few miles per day just doing the activities of day to day life. That's right a few miles. Which means I'm getting quite a bit of exercise without any real extra effort.
** Human's don't have tailpipes but where we get our "fuel" is still important. Farming in our country is very much tied to fossil fuel use. Buying local and organic can greatly reduce the environmental impact of our human fuel intake.