Don't let these curmudgeons discourage you...go for it! Don't worry about the spanish, by the time you're halfway across Mexico you'll have picked up enough to get by...and after a year? You'll be fluent.
Check out the guys at
http://ridingthespine.com/main.html . They're taking the "scenic" route to Tierra del Fuego from Alaska. They seem to be doing fine...and having a blast no doubt.
Any undertaking that large is going to be filled with difficulties and unplanned events...an open attitude and willingness to adapt are key. Keep us in the loop!