By the time Mary becomes Queen, she'll be closing on 50. Maybe even after. This is not what she hoped / planned for.
I'm still waiting for the announcement that Fred will give about the crown and kingdom being a too heavy burden for him and because of this he'll refuse to become king and gives the crown to his son, King Christian XI, who will by then be prepared and a grown up man in his twenties at least.
Not-king-Fred will instead take a loong loong holiday with some young blondes, while failed-Queen-Mary books a another trip to Switzerland.
She'll just be Queen Mother Mary and probably won't even care about Fred at that point.