One of the early signs of fatigue in running is your inability to have sufficient separation between the thighs low low low during the pushoff. This is usually caused by a weaker pushoff ( less powerful ankle joint extension) and a weaker forward knee drive. The weaker knee drive results in less distance that the thigh is brought forward which, in turn, creates a shorter stride length. Because you cannot raise the thigh in front of the body as needed over the entire distance, your running becomes slower. To correct this, you should do the knee drive exercise for strength endurance. This exercise is extremely important for strengthening the hip flexor muscles that are responsible for driving the thigh forward on each stride. The knee drive exercise is best done with the Total Athlete System so that you can duplicate the same neuromuscular pathway seen in the actual run. When you begin with the thigh as far back as possible, so that you feel a stretch of the hip flexors, you also develop strength in the same range of motion as in the competitive run. When you do specific exercises that duplicate the joint and muscle actions seen in the run, you can go the distance without undue fatigue and thus achieve your goals. The strength endurance that you develop, when you do high numbers of repetitions, will enable you to continue running with the same form for the entire distance. This is the key to success not only in the short distances but also in long-distance running. See Explosive Running for more details.