A 2-hour easy run is simply not enough if you have to be out there for 3:30 on race day. So the walk/runs is an easy way around this. So these runners would break for mechanical reasons and not for endurance reasons.
-Marius Bakken describing his marathon training program. He advocates walking during the long training runs but not during the race.
It looks to me like he's got some pretty good ideas that make sense for the average runner- you can order his 100 Day Marathon Training Plan for $47.