I don't know about coming from Manhattan, but Atlantic Avenue in Brooklyn turns into North Conduit, which then turns into Sunrise Highway, and that goes pretty much the length of the Island. It's not a real highway, just a main road through lots of boring suburbs (I've ridden on it in Nassau County a bunch). Not the most scenic route, but good and direct to get you out of town, and not too scary for riding. (Also, no shortage of convenience stores in case you want a Slurpee or whatever.)