How a Walk in the Woods becomes a road book

The first work I read by Bill Bryson was The Lost Continent, a true road book in that he has a car and takes to the road.  But A Walk in the Woods is more sensitive, more interesting and although it takes place on a trail, it is a big trail; - The Appalacian Trail.  And since so many roads started out as trails, this is deserving of road book status - and of being on everyone's must-read list.  Caveat Emptor, I shall reveal my bias; I love this book.