Shanks' Mare Meaning

(US, idiomatic) One's own legs used for walking; to "travel by shanks' mare" or "ride on shanks' mare" is to walk to your destination.

Example: 1942. Transit Journal, McGraw Hill, page 173[1]:
  Once they take to Shanks' mare, they are all equal
1869. Iowa newspaper, The Dubuque Daily Herald, May 1869:
  A public exhibition of the velocipede [a predecessor of the bicycle] ... will never come into general use in competition with Shank's mare.
2003. John O'Flaherty, Rafferty, p. 77[2]:
  The last thing we'd want to be seen using shanks mare, even though the day is holding up well.