When using Google Maps, you can modify the route if you don't like what Google comes up with.
If you look at the highlighted route, there are white dots, grab and drag one to the route you want and it will redo the route. Sometimes you may need to do 2 or 3 if it keeps trying to push you back to a freeway route (the routing logic give preference to staying on major roads).
We've never had an issue with the route lengths being incorrect. Do make sure you are comparing apples to apples. If you just enter the city names on Google Maps and put in the exact street address on your garmin, it will come back with different results because you asked for different start and end points.