Well, I could easily change This Land Is Your Land to a waltz, and that's a very traditional pattern too. You might object. Lord Marlborough is an English ballad, not a country song, and is well established in 5/4. And that's how the source singer sang it.

I agree that it's in C, but there are two very strong Bb notes half way through the 3rd and 4th lines which define the character of the tune.

The printed source, Frank Purslow's "The Constant Lovers," does give guitar chords, so I'll give them here. (That's not a recommendation, it's what's there.)

Am/C | Dm/Gm | Am/G7 | C |
C/Am | F/G | C | C |
Am | F/G | Am | F |
C | Dm/Gm | Am/G7 | C ||