Other than a rough swath... I don't bother with an exact route..
Just make a rough plan every night or morning on where you want to end up the next night.
Figure between 100 and 200 miles per day on average.
It will vary day to day depending on your mood and weather and schedule.
Pick a likely town on a map that night or morning... and take the slow speed roads getting there.
There are almost always slow speed roads parallel to high speed roads.
The real question is ... motel only?... or tent and sleeping bag also?
That affects the trip greatly.
Planning too precisely is a waste in my mind... there are too many details that change daily.
A good sense of direction is necessary for my method.
As long as I am heading West (and my objective is West!)... I am happy.