I tried something similar (Alaska to Tierra Del Fuefo) following the Pan-American Highway. However, I couldn't get google maps to work out the mileage/route because there is a 90km section between Panama and Colombia (the Darien Gap) that does not have any roads and is almost completely impassable.
I therefore settled on a much shorter, less ambitious route (John O' Groats to Land's End).
Good luck with your walk. Hope you get to Macchu Picchu one day.
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