You can get a GPS (mine has the choice of "auto" or "bike"). I like & use Garmin; they make a great handlebar mount for most of their models.
If you don't want to spend any money, you can use Mapquest.com (check to avoid highways and toll roads) or Googlemaps.com (pick "walking"). They will let you plan the routes, but they won't properly calculate how long it takes. You'll have to figure that out by your average speed and how long you'll ride without major breaks.
