So just to have an idea, I invite you to have a look on the map. I called this map "El Sur" (= South in Spanish)
Starting from Whitehorse, Yukon.
Going by Vancouver, B.C,
Victoria, B.C.
Aberdeen, WA.
(May be Portland, Or.)
San Francisco, CA.
Los Angeles, CA.
(I'm not sure if it's better to go by the peninsula of South California and after to take a ferry, or if it's better to go by Arizona).
I was looking for this ferry from La Paz to Topolobampo, but it's very expensive... like 850$ ! So.. Still looking an other solution, may be hitchhiking boats.
So for the next cities I have no ideas. But I'll probably cross those countries :
Guatemala, El Salvador, Honduras, Nicaragua, Costa Rica, Panama, Columbia and finally Ecuador.
This project of itinerary is approximatively 13.000km.