Central and Prospect parks. I moved last week to hoboken and there is nowhere to ride, but I used to live on central park west and used to get up at 5:45am and do 6am group rides. Can easily do 14 minute laps (26mph or so avg) in pacelines and it's safe. I think there are some pros that train in prospect at 6am because they live there but it gets monotonous. I like river road / ross dock picnic area over the GWB and hills in nyack are steep if you want to go for a long ride. You can also just take the A train to 178th street so you're not riding 20+ miles through BK and MNH, then you can get off the subway and have a good ride with varied terrain in NJ/NY.