The run-walk method doesn’t mean you just slow down and walk when you’re tired, it means “taking brief walk breaks when you’re not”, which “gives muscles regular recovery time during a long run”. The key word here is brief. I think the suggestion of a 1-minute walk per mile or couple miles is ideal.
So what's your take? Do you think incorporating some walking into your running is cheating? Or a life-save? With my bad knees, I think this plan could prevent me from a lot of pain. You can read more about the run-walk training technique here.