I think this is the ultimate Manhattan walk - 20 km or so, taking in Union Square, Midtown, the Upper West Side, Central Park, Harlem, the Upper East Side, the Metropolitan Museum of Art, the East Village, Chinatown, the Financial District, and finishing up within sight of the Statue of Liberty.

I got the taxi up to Union Square around 9:30 am, then walked up Broadway through Midtown. There were neon signs and it was starting to get busy. Then I walked all the way up the west side of Central Park on the Upper West Side and enjoyed the trees covered in frost. At the top of Central Park, I turned east and walked through Harlem, the most famous historic African American neighbourhood.
At the northeast corner of Central Park I turned south, walking down Fifth Avenue, alongside Central Park. I walked down to the Upper East Side and spent a little time in the Metropolitan Museum of Art. Then continued south, past the southwest corner of Central Park, and back into Midtown. By now Midtown was full of crowds of shoppers and tourists!
I carried down until Fifth Avenue reached Broadway again, then walked down Broadway back to Union Square.
From Union Square I followed Bowery southeast, through the East Village and into Chinatown. Chinatown was packed with people - I really enjoy Chinatown because it is like a city within the city. Then I walked under the Brooklyn Bridge, and into the Financial District, which was empty because of the weekend. I walked all the way down to the ferry ports and sat in the park at the southern end of Manhattan, watching the boats come in and looking at the Statue of Liberty. I finished around 2 pm.
I think I walked about 20 km.
And I feel good about it!

Midtown and Chinatown were both crowded. I love Chinatown because it is so interesting, but really don't like Midtown because it's just full of shoppers. Harlem is interesting too, and walking by Central Park is very relaxing. The Metropolitan Museum of Art is one of the great museums of the world, of course.

.