The entire last stanza of the poem is a big farewell. The speaker is going somewhere, and it's not clear where (here's hoping it's Vegas). He makes it seem like he won't be back for a while; he says farewell twice, then says he will come again, even if he has to walk ten thousand miles. The concluding farewell makes the poem just a little bit sad; after all, when people are in love it's never fun when one of them has to leave for a while. But we're holding out hope that the rumors are true—absence really does make the heart grow fonder.
Line 13: The speaker says, "fare thee weel" to his "bonnie lass." Wait, where you goin', dude?