Distance workouts for interval training resemble distance workouts for base building.  The distance day cycles from a moderate distance and builds to a longer distance, with the longest walk no farther than the greatest distance walked during the base building phase.  The cycle repeats with each cycle increasing your workload for a few weeks, backing off, and then increasing it again.  Importantly, your distance days need to be at an easy pace, so do not increase the pace of the distance workout from week to week. Instead, increase your pace at the start of each mini-cycle and maintain it throughout that cycle. As the length of your easy distance walk increases, your weekly workload elevates slightly.  While accelerating the pace of your distance workout at the beginning of each mini-cycle adds to your workload, mileage reduction associated with the beginning of each mini-cycle tempers this acceleration. The true increase in workload comes from the other workouts added to your training regimen.