Here's my fitness ride that I do for cross training with running. I did it today. I love this quick 12.5mile route from east Williamsburg to downtown brooklyn over the manhattan bridge through soho/chinatown back over the Williamsburg bridge loop. Take note: it does transverse the "great hipster silk route" twice. But its mostly commuters on the time I ride it (either early morning or after 5pm). You can get fast on the bridges and in downtown brooklyn/williamsburg, but you also have to deal with chinatown and traffic lights.