Origin of the term 'country mile'?

Solution 1:

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.

Solution 2:

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.

Solution 3:

A country mile is perhaps a far longer arbitrary distance than a proper mile for the considerably longer distances between homes and other settlements in the country than in less rural areas.