Changing speeds during an interval running workout can help build race ready fitness in a manageable, less destructive way than just straight up hard, fast intervals.  Lactate Threshold (high-end AEROBIC pace) 1200 meter intervals tire the legs sufficiently - mimicking racing fatigue without being abusive, followed by some shorter, faster intervals 300 meter / 200 meter / 100 meter sprints, challenging the body the same way as the end of the race.  

Amy running lactate threshold pace followed by sprints