Yes, it was Joseph Mitchell. He was a great writer. They published part of an unfinished memoir he had been working on about his visits to all of the city's neighborhoods: http://www.newyorker.com/reporting/2013/02/11/130211fa_fact_mitchell
I've covered some logistical info on my site -- http://imjustwalkin.com/nyc-details/ -- but here are some answers for you:
I plot the routes by eyeballing my progress map and trying to quickly (usually right before I walk out the door) come up with something that covers new ground and doesn't leave weird little isolated unwalked stretches that will be hard to get to later. I've just been couchsurfing around, so I don't usually know where I'll be staying too far in advance, which means I have to be flexible with the route planning.
I've had a good number of people walk with me, but it can be tough to coordinate because all my planning is so last-second.
I have at least two more years of walking ahead of me on this project, but I wouldn't be surprised if it ends up taking longer.
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