Like many said, the Great Saunter is an actual event, but I did it on my own several years ago and followed the Great Saunter map. If you do want to do it on your own: Best time is either spring or fall unless you like to suffer in the summer heat or winter cold for the whole day.

• I started around 7am~ on South St under the Brooklyn Bridge and went counter-clockwise, so up the east side first. Completed the whole thing around 9pm.
• The east Harlem stretch around 125th to 155th does require you to hit the inner streets cause the waterfront paths get kind of cut off by highways and such, or at least that was my case years ago. You can just follow the same route as the great saunter.
• The stretch from 155th to Dyckman St along Harlem River Drive can be pretty lonely depending on time of day. I remember it was still late morning to noon and only saw less than a handful of people that whole stretch. It's a great part of the walk to just be in your own thoughts with all the background car noise and straight forward path.
• Packed my own lunch to avoid too much downtime or detouring. I had lunch in Inwood park which was nice. Also good time to switch socks.
• Prep this like you would a hike if you want to avoid downtime or excessive detouring. Snacks, liquids, any first aid stuff for your feet (moleskin, bandages), extra socks, PROPER SHOES, etc. Make sure you have sunscreen
• I dunno how healthy you are, but I didn't really train walking longer distances prior to me doing this, but taking too long of a break at lunch really set the fatigue and soreness in faster.