NEW YORK (CBSNewYork) — Step by step, block by block, borough by borough — 69-year-old William Helmreich has walked them all.

“If I had any idea how hard this would be, I never would’ve done it,” he told CBS2’s Scott Rapoport.

An astounding feat by any measure, the sociology professor at CUNY says he’s hoofed all 120,000 blocks and 6,000 miles of New York City, virtually every square foot.

It took him four years.

Crazy?

“It is a little meshuga,” Helmreich said. “But in order to do something interesting, you sometimes have to be crazy.”

That raises the unavoidable question: Why?

“I do it because I love the city,” the lifelong New Yorker said. “The city’s fascinating to me. It’s the world’s greatest outdoor museum.”

Helmreich said he performed his pilgrimage through a variety of meteorological miseries — sleet and heat, rain and snow, taking it all in stride.

“Just awesome. It’s unbelievable,” he said. “You’re walking in the street. There’s nobody around. You say, ‘What the hell am I doing this for?'”

The results are a book he’s written called “The New York Nobody Knows” and a seemingly endless array of enviable experiences: the tree with 1,140 stuffed animals in Bergen Beach, Brooklyn; the school bus limousine in Sunset Park, Brooklyn; an ultra-exquisite sunset in Battery Park City; and a view of the gorgeous mosaic that is New York City.

“Every day, I learned something new,” Helmreich said. “Every day, I saw something I didn’t know existed.”

He said the craziest thing he ever saw was a man in Bushwick, Brooklyn, walking down the street with four pit bulls and two boa constrictors wrapped around his neck.