The Interval Training Phase is composed of two mini-cycles. As you progress through each mini-cycle, the weekly workload increases. Then, after three weeks, you back off and recover for a week. Throughout the phase, workload intensifies in three ways. First, the distance workouts grow longer. Likewise, the tempo workouts increase by a half mile each week during each mini-cycle. The final increase occurs in the form of longer intervals. You still walk the same distance in total mileage, but each week the individual intervals lengthen.
Your workload also builds from mini-cycle to mini-cycle. The second cycle mirrors the first, just a bit faster. After adapting to the stress of the first mini-cycle and then resting, your body is ready to accept more stress in the form of an accelerated race walking pace.
If you have problems finishing an interval workout, do not quit. Instead, consider two options. Either slow down to a comfortable pace you can sustain for the remaining repeats, or shorten the distance of each repetition. Selecting the latter option requires offsetting the dodged workout by completing additional intervals; this way, the total distance equals that of the original schedule. Finishing these difficult workouts trains your mind to focus and endure, providing great mental advantages when difficulties arise during a race.