If you want to race at the same pace as your easy distance race walks, then training at your easy distance pace most of the time is fine.  However, most people want to race as fast as possible, so their training needs to incorporate faster walks.  Race walking faster produces waste products that the body may or may not be able to handle.  When your race walking pace remains at or below a certain level—the lactate threshold—then your body can process one waste product, lactate, at the same rate it produces this waste. When you race walk faster than your lactate threshold pace, more and more lactic acid builds, eventually slowing your pace.  Therefore, to improve performance, you need to train to increase your lactic acid threshold.

The Interval Training Phase includes workouts that train your body to improve its lactate threshold.  Pushing the threshold higher enables you to race faster without overloading on lactate.