The area is totally walkable. The Yellow / Orange road show on Google map are road / main road, while the one in green are highway (normally above-ground for Tokyo), google map shows pedestrian path if you zoom to the closest level in dotted lines. It's nothing to worry about.
http://local.google.co.jp/maps?f=d&s...0&ie=UTF8&z=19
Personally I will take Narita Express to Shibuya (using foreigner discount package) + Tokyo Subway Ginza Line even flight got delayed.
Not changing Hanzomo line @ Tokyo station because there's quite a bit of walk from Tokyo station to Otemachi