Hi Max, I suspect that your walking route is a little bit longer because you can't walk on highways but anyway it is an interesting virtual tour. I should suggest you to change the route to visit some famous USA places as Death Valley, Mount Rushmore, Golden Gate Bridge and so on and then when you will do the equivalent distance you should make a "virtual photo" of you in the place, so you will update us of your progress with something like a virtual postcard. :-)
Curious. Interesting that in the US the walking distance is longer than the driving distance, which does not match my quick tests in Australia, the UK and Europe, presumably because a driving route is optimised for speed, not distance and walking is all about the distance, including any suitable back road.
The US must be really optimised for driving!
Google keeps offering the use of ferries in Europe for walkers, though. London...Edinburgh takes you via the Netherlands...