Our word "mile" comes from the Latin "mille," which referred to the Roman mile. The Roman mile had military origins, since it was the equivalent of one-thousand double paces of their marching soldiers. The soldiers' double paces were about five feet, so the Roman mile was about five-thousand feet.
Since we got our measurement system of inches, feet, yards, and miles from the British, what does the Roman mile have to do with our mile? Well, Britain was part of the Roman Empire from the first to the fifth centuries A.D., so when the British began to standardize their measuring system there was a Roman influence.
Even before the British started keeping written records of land holdings, the farmers laid out their fields in plowed furrows that were consistently the equivalent of a modern 660 feet long. This distance became a standard part of their measurements. Over time, by slurring the words, this "furrow-long" distance became "furlong," a unit that is now used almost exclusively in horse racing.
The British eventually used the Roman mile as a model in their measurement system, but they didn't want to give up their furlong. The Roman mile was about seven-and-one-half furlongs, and when the British adopted it, they lengthened the Roman mile to eight furlongs, which equals 5,280 feet.