"easy victory," 1838, such as one that happens in the absence of competitors, when the solitary starter can traverse the course at a walk. Transferred sense of "anything accomplished with great ease" is attested from 1902. To walk (all) over (someone) "treat with contempt" is from 1851.
Examples from the Web for walk-over
At first thought it seemed that Goliath would have a “walk-over.”The Columbia River|William Denison Lyman
It looked like a walk-over for Clarence, but in the fifth round he blew a couple of green notes and Lancelot got the decision.You Should Worry Says John Henry|George V. Hobart
As he sat down, I realized that it was not going to be a walk-over.Berry And Co.|Dornford Yates
The expression is used advisedly, for it was nothing but a "walk-over."Destruction and Reconstruction:|Richard Taylor
With him out of the way it would be a walk-over for the Sox.Baseball Joe in the World Series|Lester Chadwick