I apologize if this is old news, but I was unaware of it until recently and it's quite helpful.
If you go to
http://maps.google.com, click "Get directions" and enter your starting point and destination, it will default to the shortest slab route.
However, there's also a check box for "Avoid highways". This will almost certainly yield a better, if longer, ride. So give it a try if you're not in a hurry.
