Here in Ontario, Canada I found that I have to be careful when planning tours and using Google maps to do so. That's because Google maps will often divert you several kilometers/miles to put you onto a trail that you then takes you several kilometers/miles out of your intended straight line route.

I use other sources with Google maps.

Btw, one thing that would be very useful in planning a route would be something that let you know the different gradients of nearby roads. I've ridden my bicycle around here and sometimes on road will have very steep drops and climbs whilst the next road over will have gradual drops and climbs which over a day long ride saves a lot of effort and energy especially if the bike is loaded.

I also like topographical maps, small dividers 9for calculating distance on the map), an orienteering compass and a bicycle computer that gives distance. With those items I can stay found and explore a lot of unpaved back roads.

Cheers