It seems to me that if I run up to the George Washington Bridge, then back down all along the west side to Chambers Street, then back up to home at 116th St., this gives me roughly a twenty-mile run without the problem of having to repeat a loop or come temptingly near to my apartment at some not-far-enough-along point in the mileage. I will be doing it solo, so it will be a good mental test - I will hope it is not as rainy as it was today, though!
[ED. Afterthought: it divides naturally into thirds, too: roughly 6 up to the bridge and back down to where I live/the entrance to Riverside Park, then 6-7 to Chambers and 6-7 back up. So I can start out easy and run a little faster on segment 2 and then try and hit marathon pace for segment 3.]