Google on the phone sucks for that, you can't do it. You have to put together a system of software from other people to make it work. The only way I know how to do it requires a Windows PC and an Android phone. I know my method won't work on an Iphone and I don't know of any methods for that. There might be, just dunno.
Plan the route on Google Maps. Put in lots of waypoints to force the route to be on the roads you want. The more the better because we are going to translate that twice.
Download ITN Converter from the link below and put that on your PC. Copy the full URL from the Google map you made above and paste it into ITN. Then there is an export button that will convert that to a file for your phones Navigation program. It supports most programs. It is free.
http://www.benichou-software.com/index.php?option=com_content&view=article&id=4&itemid=3&lang=en
Get a good Nav program for your phone. I use Sygic. It is cool, you can download maps for the whole US...state by state so you just get the states you need. Then you copy the file from ITN to your phone and open it with Sygic. Sometimes it takes a try or two to get all the routes/waypoint right. Sygic is like 25 bucks for all of North America. There is a free version you can try for a week or two.
https://www.sygic.com/gps-navigation