What are you measuring? ...
The orignal definition of a meter (and thus kilometer) is based on the size of the earth: "one ten-millionth of the length of the Earth's meridian along a quadrant; that is, the distance from the equator to the North Pole".
The orignal definition of a nautical mile is also based on the size of the earth: "one minute of arc of latitude measured along any meridian, or about one minute of arc of longitude at the equator".
Both of these are fairly well know derivations.
However, many may not realize that the mile is not arbitrary, but is based on the length of a man's pace: "The Romans were first to use the unit of long distance mille passuum (literally "a thousand paces" in Latin, where each pace or stride was two steps). It denoted a distance of 1,000 paces or 5,000 Roman feet, and is estimated to be about 1,479 metres (1,617 yards). This unit, now known as the Roman mile,[12] spread throughout the Roman Empire, often with modifications to fit local systems of measurements."
A modern mile is a little bit longer than the old Roman mile, but not by much. I occasionally count paces as I walk long distances, and find that walking briskly on level ground, my pace is almost exactly 1/1000 of a mile.
Anyway, not that it matters, but if you want to know how far you've gone realative to the size of the earth, use nautical miles or kilometers. If you want to know to know your distance relative to going on foot, use miles. Then computer your speed -- knots, mph, or kph.
Note: All quotations from Wikipedia.
