Walk breaks taken early in the run keep the muscles strong and resilient enough so the legs can move with strength and efficiency throughout. This will significantly reduce or eliminate the excess stress around the knees, ankles, feet, etc. which produces injury. The little “back-up” muscle groups can stay in reserve and fine-tune the running motion after fatigue sets in.
The earlier you take the walk breaks, the more they help you! To receive maximum benefit, you must start the walk breaks before you feel any fatigue in the first mile. If you wait until you feel the need for a walk break, you’ve already reduced your potential performance. Even waiting until the two-mile mark to take the first one will reduce the resiliency you could regain from walking in the first mile.