Ive always known Loch Lomond to be about two soldiers (Scottish) who were captured in England and one of them dies. The low road being his death and the high road being the one to Scotland. and i'll be in Scotland afore ye... implies that he'll be there in spirit before the other one escapes over the border. Its always been this story in my house. Not sure if its true or not, but the folk in my town have always believed it!