A tall, lanky, rather dark Black person (hence the term, Monday) who partakes in a 26.2 mile long running competition. They are typically, but not always, imported from Ethiopia and Kenya.
White Guy 1: "Hey, Murphy, who's that Black guy? Is he on the Celtics?"
White Guy 2: "Nah McDonough, today's the Boston Marathon; it's one of those Marathon Mondays."