If the earth was flat then surely you would reach an end rather than end up at the same place again if you keep on going long enough. There would have to be an end point rather than the ability to go round and round and end up in the same place. When Palin did Pole to Pole he pretty much documented the full journey. There was no end point where he fell off the earth. You can prove he did it by copying the journey yourself which others have tried to do. If the globe he used to illustrate his journey was cut up and laid flat he could not complete the journey without going back.