I wanted to do this so badly after doing my first century last fall however with how awful our winter was, I just didn't have any time to put in training and I really wasn't looking forward to suffering and getting my ass totally kicked. I wish it was a little bit later in the year so some training could be done. I'm in fantastic cycling shape now and would take on the challenge in a heartbeat if it were now.
IF you want a GREAT ride around the city, you should consider doing the NYC Century. It's coming up on the 18th of this month and it's a great ride through 4 of the 5 boroughs. its virtually flat except for the times you go over some bridges but it's awesome the way you go through all the different neighborhoods of the city. I really enjoyed it last year and am doing it this year. Another good ride similar to Gran Fondo is New York Cycle Clubs Escape New York. It's only about $40 which is nice and goes from 125th street into NJ and back up into new york looping around and coming back down along the hudson. there are some climbs but nothing too major. That ride is the week after the NYC century on the 25th. I plan to do both.