This will get you north with minimal motor traffic interference. And it might sell you on the benefits of rail trails.
Consider stopping at the Country Deli in Millwood. They bake their own bread and it is a mere block to the left when you cross Rt 120.
You can make it a "mostly rail trail century." Start in Manhattan, on the Hudson River Greenway up to Van Cortlandt Park (see directions on http://nycc.org/node/48587), Continue on the Dutchess Rail Trail over the high bridge and follow to the end at Tony Williams Park, take a left onto South Riverside Rd, the first right on County Route 12/New Paltz Rd, and a left in a mile onto Rt 299 west. Follow to New Paltz, then turn around and catch the train in Poughkeepie.