It's a half marathon's distance as the bird flies from JFK to midtown. That's almost four and a half hours at a twenty minute pace. Not walkable for anyone but an athlete.
Barring something like serious obesity or disability it’s impossible for me to imagine an adult who couldn’t walk for for and a half hours, especially on flat terrain. That’s basically an easy hike.
The first time I went to the Netherlands I walked from Schiphol airport to the center of Amsterdam and it was an easy but amazing experience. Walking from JFK to Manhattan is a completely possible walk in terms of distance (I usually do better than a 20 minute pace on the flats so it takes only half a day) but when I think of the battle with the infrastructure you'd have to do it's like an Arnold Schwarzenegger movie.