You might want to consider Northern Blvd in your plan. I know it extends well into Long Island and runs through Queens and will put you very close to the Queensboro Bridge. From there, I'd do what others said and cross the GWB. When you cross from Queens into Manhattan, you can head west and get on the West Side Bike Lanes and ride them north until you are in the GWB area. There is a pedestrian overpass right on the lane in the area that would make your exit from the west side lanes and onto the streets, where the bridge is a few blocks away. It will be obvious because you can see the bridge from the lanes. Can't miss that overpass.