I was looking at a Google map and it appears they now give directions for walking. Just for fun I switched from car to walking, this is what I got. I could walk from Michigan to New Mexico in 22 days and 6 hours.
Walking directions are in beta.
Use caution – This route may be missing sidewalks or pedestrian paths.
Walking directions to Williamsburg, NM
– about 22 days 6 hours
On the positive it is ever so more direct than the car route.
In the documentary Roam Sweet Roam there was a woman who was walking across the United States with a cart a tent and her dog. Think just how fit I would be. Maybe someone could deliver the Airstream for evening stops.