The Zumo uses Mapsource, so yes, you can plan a route that uses your preferred road. The 'Avoid Interstates' function on the route preferences will not absolutely eliminate interstates from its' on-board routing.
You need to remember that if you don't use 'hard waypoints' to tag your preferred roads, if you go off-route, the unit will automatically recalculate a new route for you and it will ignore your preferred roads altogether.
One of the ways that I've found some great roads is by telling the GPS to find the "shortest" route, rather than the "fastest." You do, however, have to be ready to turn around or ride some gravel once in awhile... :lol:
The GPS will recognize your preferred route if you have programed it in MapSource using enough waypoints. This means you must tag your route with the flag type waypoints to force your preference. If there are not waypoints for the Zumo to honor it will recalulate the route using one of the 3 available options: Faster Time, Shortest Distance or Off Road.
If a resulting recalculation puts you on a route you don't care to ride you can always add waypoints to the route on the unit to force your choices. This operation can be quite time consuming, especially if the route is long and complicated.
Some of the other Garmin units allow you to specify the kind of roads you want to navigate under a Custom Preferences menu item in the route calculation choices. I can't understand why Garmin left this option off of the Zumo.
Jon's Zumo pages may be of some interest to you, here is the link: