Walk breaks keep you from using up your resources early. By alternating the exertion level and the way you're using your running/walking muscles, these prime movers have a chance to recover before they accumulate fatigue. On each successive walk, most or all of the fatigue is erased, bestowing strength at the end. Frequent walk breaks also reduces the damage to the muscle dramatically, allowing you a faster recovery afterwards.
When a muscle group, such as your calf, is used continuously step by step, it fatigues relatively soon. The weak areas get over-used and force you to slow down later or scream in pain afterward. By shifting back and forth between walking and running muscles, you distribute the workload among a variety of muscles, increasing your overall performance capacity.