Wow, that one guy totally insulted Lydiard by comparing him to Galloway. Don't do that, please. Galloway's methods are a money making public relations stunt designed to appeal to beginning runners. Lydiard's methods are the real deal ways for a dedicated runner to achieve optimum performance.
As for what Lydiard said about a 20 second break, yeah, walking reduces your heart rate almost instantly and begins to give you less of a workout. It alters how your body takes the total stress of the run. The more you walk, the less you stress your body. That's great if you can't handle the distance you're trying to cover, but pretty unhelpful if you know what you're doing and you pick training workouts that optimize your running ability and allow you to actually practice running for the whole duration.