Random piece of general knowledge (many thanks to The Old Farmers Almanac):
1 league = 3 miles = 24 furlongs
In other words, there are eight furlongs to a mile. So how long is a furlong? 660 feet, or 40 rods (one rod being 5 ½ yards). Seen another way, a furlong is equal to one eighth of a mile: equivalent to 660 feet, 220 yards, 40 rods (1 rod = 5 1/2 feet), or 10 chains (one chain, therefore, being equal to 66 feet).
Originally, it was the length of the furrow in one acre of a ploughed field – thus, the name: one “furrow long” – in the old open-field system of medieval England, in which acres were usually long and narrow, and was the distance a team of oxen could plough without resting. From there, it passed into the British Imperial and U.S. customary system of measurements. An acre was reckoned as one furlong in length (naturally), and one chain in width, and was considered to be the amount of land one man, behind one ox, could plough in one day.
Other oxen-derived measurements include an oxgang (from the same root as our contemporary word “going,” with the implication of walking *), the amount of land tillable by one ox in a ploughing season (an area which could vary from village to village, but was typically around 15 acres), a virgate, the amount of land tillable by two oxen in one ploughing season (thus, two oxgangs), and a carucate, the amount of land that could be tilled by eight oxen in a ploughing season: equal, naturally, to 8 oxgangs or 4 virgates. Thus, these measurements were not random or arbitrary, they described what could be done on them, in a way that was very useful and informative for an agricultural society!
* That derivation still exists, though somewhat concealed by changes in the language, and our understanding: a “gang” is a group of people who go (walk) around together. And the archaic English word “gangly” refers to a person or (usually young) animal who appears to be “all legs,” and therefore seems made for walking! Also, a “chain-gang” is not just a group of people joined by a chain; they are chain-gang: that is to say, they are walking chained, rather than free.
On a related note, the furlong was historically considered to be equivalent to the Roman stade (from which we get “stadium”), itself derived from the Greek stadion ~ and it was, although approximately: the old Roman measurement was actually 625 feet. The Romans reckoned eight stadia to the mile, and (as remains the case in our English measurement, albeit using furlongs) three miles to the league. Thus, the Roman mile was a little shorter than ours is. A league was considered to be the distance a man could walk in one hour, and the mile (from mille, meaning “thousand”) consisted of 1,000 passi (paces: five feet, or two single steps of two-and-a-half feet each).
Now you know probably more than you ever wanted to about ancient land-measurements!
(Additional information gleaned from Wikipedia, and from my own knowledge of things medieval!)