There are already some great answer on when
to walk, so I'll just weigh in on how
to walk, whenever you choose to do it. It's been my experience that most runners and triathletes who spend lots of time, thought, and money on their equipment and coaching put zero thought into their walking, even if they plan to include walking into their marathon strategy. Walking is very well suited to covering a lot of ground at a slow to moderate pace; it is not well suited for moving at faster paces--say under 4 mph/15 mins. per mile. If you're a faster runner, that's a pretty substantial drop-off. Not to say the breaks aren't helpful--they are--but wouldn't it be great to be able to walk comfortably at 12 or 10 mins. per mile rather than 15 or 18? I coach racewalking, but I'm not going to got there. I will, however, suggest adding just a few elements of the Olympic technique if you want to pick up your walking pace without increasing your risk of injury.
Whether walking or running, your speed is a product of stride length and stride frequency. To go faster, increase both, right? Until you try it... You'll soon find that trying to maximize your stride length inevitably results in a reduction in stride frequency, and trying to maximize stride frequency results in a reduction in stride length. The key is to find ways to increase stride length without inordinately reducing stride frequency, and vice versa. Here are three:
1. Use your feet for a longer more powerful stride! Most people simply pick up their feet and plop them down again when walking. But active feet can be a great source of propulsion. Land on the heel, then as the foot begins to drop (plantar-flex) begin pushing through the ball of the foot, after the body starts to pass over it. Continue pushing all the way to the tip of the toes. Practice while walking around during the day. Pushing against some resistance (like when pushing a full shopping cart, for example) gives a good demonstration of how much power you generate by simply using your feet.
2. Bend your elbows! A long pendulum is a slow pendulum. Swinging the arms fully extended or nearly so, results in a pendulum with a very long period
--the amount of time it takes for a pendulum to swing out and back. Simply bending the elbows near 90 degrees will result in a faster, more powerful swing. Focus on driving the elbow back behind the body, which will help send more power to the foot pushing off on the other side of the body. (Driving the right elbow back will help help with toe-off on the left foot.) Keep the arm swing short in front of the body to prevent over-striding, which puts the brakes on forward momentum.
3. Think shorter/faster rather than longer/slower. Using your feet and driving your elbows back behind the body will help to create a long powerful stride behind the body. Your job now is to increase the number of those strides per minute by focusing on fast feet. I'll often tell athletes to think long and powerful with the arms, short and fast with the feet.
If you want to get down below 9-minute miles I can get you there, but that's going to require actually racewalking. (And I'm not going to go there!) ;-)
America's only 8-time Olympic Track & Field Trials qualifier http://www.racewalking.org