Is it an old farming metaphor, or a military saying? Where did this(these) saying(s) originate?
The original form of the phrase appears to be "a hard row to hoe". Now, "tough row to hoe" is found at least as far back as 1890:
She's got a tough row to hoe, Dilly Gage has. She used to try to keep folks from knowin' how cantankerous he was, but she couldn't.
while 1963 seems to be the earliest occurrence of "tough road to hold", and it is noted as an error:
Certain little errors may be the result of spelling, hearing, or carelessness - "a tough road to hold"
He loves to contend with difficulties; and if he had not a hard row to hoe, would place himself in some sphere demanding effort, in order to extricate himself.
Since it's used without explanation in the 1818 book, it must have been regarded as a well-known expression even by then. So the origin is as a farming metaphor, and seems to be chiefly American in usage.
A "tough row to hoe" does in fact come from early American farming in days with limited machinery - in other words they used a hoe in the field to pull weeds from rows of crops. It was hard work done by strong men in hot fields especially across the South. In talking with my father who actually did some hoeing when he was a kid, he thinks the saying comes from the so-called "down row." There was some equipment used in the field on certain crops which caused the furtherest row out to be damaged somewhat and knocked the crop down. When hoeing, you work hard to pull weeds and not damage the crop itself. So, when men hoed a field ther may be 15 or 20 workers all of which take a row which is theirs to hoe. When the worker reaches the end of the row and has finished, they walk down to the next row that does not have a worker assigned to it. The men wanted to avoid this so-called down row as they went to their next row becasue it was -- you guessed it, "a tough row to hoe."
Frankly, it bothers me to hear the unknowing say "a tough road to hoe." No one hoes roads other than convicts. Men hoed crops.
The expression appears to be American. Frontiersman and Congressman, Davy Crockett (the "King of the Wild Frontier"), used the expression in his book, An Account of Col. Crockett's Tour to the North and Down East (1835).
The two earliest examples I could find are both from the same magazine in 1823, and suggest that the expression had already moved into idiomatic use by that time. Although most of the early examples of the idiom I have found refer to a "hard" row to hoe, even the now more familiar "tough" row appeared in 1823:
"[I]t seems that the French have rather a tough row to hoe, and are likely to get more kicks than coppers, to pay for their toils and dangers in Spain." (Italics in original), New England Farmer, Volume 1, Number 48, June 28, 1823, page 383.
"Still the Greeks have a "hard row to hoe." The Turks are collecting fresh armies, and threaten to overrun the Peninsula." The New England Farmer, Volume 1, Number 40, May 3, 1823, page 318.
I believe the road/hoe, and the road/hold sayings are the results of mishearing, or purposely changing the original row/hoe saying. Similarly, on occasions, I have changed the expression "don't look a gift horse in the mouth" to "don't lick a sick horse in the mouth" just to get a laugh. However, we all know the original meaning.