There are several coaches who are strong believers in walking intervals (during training and races):
- Joe Henderson (10:1): http://www.running-world.net/walk_and_run_better_1.php
- Jeff Galloway (3:1 or 2:1 or 1:1): http://jeffgalloway.typepad.com/jeff_galloways_blog/2010/03/index.html
For example, in 1984 Stu Mittleman set an American distance record for the 6-day World-Championship endurance event, 578 miles. He did it by following Dr. Philip Maffeton's advice and alternating 1hour walks and 1hour runs, and was still going strong on the last day when many others had long-since hit their "walls".
I believe that the "Running Room" programs use a 10:1 Run:Walk approach.
In particular, see the charts in my Warmup post to see how such Run:Walk intervals help me lower HeartRate (and Lactic acid and stress/injuries) throughout a run. I now use this approach during my Long-Slow-Distance weekend run, but not during my slightly faster mid-week speed/strength building runs (where I intentionally want to stress the muscles a little more).