Yes. It is from Intervention. In Chapter 13, he says:
"Expected = Farmer walk with total bodyweight (half per hand) Game-changer = Bodyweight per hand"
John, Dan (2012-11-21). Intervention: Course Corrections for the Athlete and Trainer (Kindle Location 1067). On Target Publications. Kindle Edition.
"Oh, and how far on the farmer walks? It depends, of course, but at least 20 meters and perhaps 40, although it would be fine to go farther."
John, Dan (2012-11-21). Intervention: Course Corrections for the Athlete and Trainer (Kindle Locations 1079-1080). On Target Publications. Kindle Edition.