One degree is 40,000 km / 360 = 111 km at the equator. But Greenwich is at 51 degrees latitude so you apply a cos (51.5°) = 0.622 factor. That's why 1 degree of longitude at latitude 51 is 69 km. Does that help clear it up?

Note also if you're walking in a circle 57 meters from the monument at the South pole, then each 1 m step you take crosses one degree of longitude. Google for: south pole ceremonial sphere