A 'country mile' is a term used casually in some areas of the English-speaking world to refer to a very great distance, but what's the origin of the term? Obviously 'mile' refers to what could be seen (from a human perspective) as a long distance, but why does the adjective 'country' combine to make it mean a very great distance?
There are two possibilities: either the difficulty of terrain makes a country mile harder to travel; or before standardisation, miles were further.
An example of the first from Frederick de Kruger's 1829 The Villager's Tale
The travelling stage had set me down
Within a mile of yon church-town;
'T was long indeed, a country mile.
But well I knew each field or style;
An example of the second comes from The Treasury of Knowledge and Library of Reference in 1850:
Robin Hood shot a full mile; and, according to his bard, a north-country mile was equal to two statute ones.
This is an interesting question. I don't have a citation for you, but I have always understood this to refer to the fact that going a mile cross country is much more arduous than the same distance over a paved road, with many more twists and turns, not to mention thickets, streams and what have you.
A country mile is an exaggerated distance. Mile is from the Latin for 1,000 paces [mille passuum] and has become standardized in English as 1,760 yards. The term "country mile" may be by analogy to a nautical mile (one minute of a great circle of the earth; fixed at 6,080 feet), an Irish mile (2,240 yards), a Scottish mile (various, including 1,976 yards), or it may be because the winding character of many country roads requires a long distance (in excess of a mile) to be traversed in order to travel between two places which, in a straight line, are a mile apart.
In the country the land is clear and you can see further. Thus, city folk can only see so far before the view is blocked. In the country this is not an issue. There is no set distance for a country mile as the distance the eye can see is variable with the surrondings. In general the eye can see further in the country hence the country mile being considered a great distance.
Things are farther apart in the country. So, if you asked me where the gas station was. I might say, "Oh, it's right down the street". Right down the street to me, might mean 10 miles away. 10 miles in the city means a completely different part of town. 10 miles in the country, is right down the street. Right down the street is a country mile.
When I hear this phrase I am always reminded of when we would play games as kids and something slightly out of the rules would occur. It was a favourite gag for the offender to grin and declare "country rules", before continuing as if nothing had happened. For this reason I have always associated "country" with "approximated and generous". Completely anecdotal and a bit of a tangent as information, but it as a country dweller the first phrase has always informed my use of "a country mile".