Many people who live in the Big Apple haven't even been to all five boroughs, much less in one day. This video of Flashwalk 2010 gives you a much easier (and at 4 minutes, faster) way to see the city.
Much respect to Kirk Larsen for reducing his legs to bloody stumps making this video.
Why is the walk so long? From the website,
The Verrazano-Narrows Bridge, which links Brooklyn and Staten Island, never had the approaches for its bike and pedestrian paths completed — something that had been promised by everyone from Robert Moses to Mike Bloomberg. This means the only walkable bridge to Staten Island is the Bayonne, originating in Bayonne, New Jersey. The George Washington Bridge from Upper Manhattan is the nearest pedestrian path to New Jersey, terminating about 20 miles north of the Bayonne.