I walked 6th Avenue today, about 8 miles, but 2.5 of them were through Central Park, so they didn't count toward the total.
6th Avenue is known as both Lenox Avenue (for philanthropist James Lenox) and Malcom X Blvd. above Central Park, where it runs from 147th St. to 110th St. through Harlem, passing many current and former landmarks.
Below Central Park, 6th Avenue runs from 59th St. in Midtown to White St. in Greenwich Village. An elevated passenger railway was built along 6th Avenue in the 1870's, depressing the value of nearby property and stymieing development. The railway was torn down in 1938 to make way for the 6th Avenue Subway line (the B, D, F, and M trains), and over the coming years it was alleged that the scrap steel was sold to Japan and subsequently made into armaments used in WWII. While this was never conclusively proven, it was never conclusively dis-proven either.
6th Avenue was formally renamed "Avenue of the Americas" in 1945 at the request of Mayor LaGuardia. LaGuardia hoped to encourage Central and South American countries to build consulates along the avenue, thereby fulfilling the dual goals of promoting pan-American trade and sprucing up a formerly gritty thoroughfare. The plan never really took off, although there are still signs along the street with the names and crests of Central and South American countries and statues honoring a number of their founders and statesmen.