Yeah thru hikers avoid roads like the plague. Judging by his route he could've walk a lot existing trails. Go southbound on Great Divide Trail and Continental Divide Trail, then somehow cross Mexico and central America into Andes, there you can follow Greater Patagonian Trail all the way to Tierra del Fuego. The European part can just reuse Trans European Alpine Route, then cross Black Sea and take the Transcaucasian Trail, afterwards maybe the work in progress Snow Leopard Track? It's gonna be a lot more difficult but definitely beats highway walking.