Back uptown again, for this walk I did the lower third of the Upper East Side ("UES"), from E 72nd St. to E 59th St; in total about 13 miles.
The UES is the eastern analog to the Upper West Side ("UWS") and is bounded by 59th St. to the south, 96th St. (approximately) to the north, Central Park to the west and the East River to the (you guessed it) east. It is home to the Metropolitan Museum of Art, the Guggenheim Museum, and 740 Park Avenue (said to have the highest concentration of billionaires in the world).
The UES was most notably home to the Gilded Age mansions of the super-rich, including the Rockefellers, Carnegies, Fricks, Roosevelts, etc. along 5th Avenue. Many of those mansions were demolished to make way for luxury apartment buildings in the 1920s, but some escaped the wrecking ball and are still around today (such as the Frick mansion between 71st and 70th, now an art museum).
That isn't to say that the UES is all but devoid of mansions, there are still a ton of them, they're just a little smaller and spread out along the streets between 5th Ave. and Park Ave. (as opposed to facing the park along 5th Ave.). Many today are owned by foreign nations and used as consulates/diplomatic missions to the UN (about two miles south).
Anyway, the breakdown of the neighborhood basically goes like this: streets between 5th Ave and Park Ave. = super rich people, streets between Park Ave and 1st Ave. = just regular rich people, east of 1st. Ave = hospitals.
The walk ended at 59th St., which corresponds with the end of Central Park and the beginning of Midtown