I'll probably end up in the spam-blocker for posting several links, but the driving vs. walking comparison has been made and criticized before. See:

http://www.stat.columbia.edu/~cook/movabletype/archives/2008/03/the_all_...

and

http://www.treehugger.com/files/2007/08/ask_the_ecogeek_6.php

with some of the key points being that people who drive, drive farther than people who walk, walk, and that if one considers all carbon wasted in our inefficient food supply system, it's certainly discouraging, but if one compares it to the carbon cost of creating and maintaining a car, the latter ends up the larger environmental loser.