To write The New York Nobody Knows, Helmreich walked 6,048 miles, covering almost every block in the city's five boroughs: Queens, Manhattan, Staten Island, Brooklyn and the Bronx. It took him four years, walking an average of 1,512 miles a year. He wore out nine pairs of shoes.
As a passionate pedestrian, I can attest to the virtues of walking cities. Heck, I've nearly killed myself doing it in the heat of Bangkok, and I've worn out good boots on trips to Paris and Hong Kong.
(Thanks to MyAppleMenu for the link)
UPDATE: The book is here on Amazon, and looks well worth a read. The cover alone is gorgeous.