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There are several things to consider. First, your start: Dick Brown, Eugene, Oregon-based coach to Olympians and author of Fitness Running, says he thinks you are simply starting too fast, which will whip you for the rest of the race. “Go out slower,” he advises.
He suggests “out-and-back” workouts, i.e. pick an approximate workout length such as 20, 40 or 80 minutes, then run out for half that time at a comfortable but challenging pace, then turn around and pick up your pace. If you feel OK in the first half of the return, pick up the pace again. The goal is twofold: one, to run faster in the second half and, two, to learn what your body can do.
Regarding your elevated heart rate, says Brown, if it’s racing so hard it feels abnormal, he suggests you see a doctor since athletes aren’t immune to “atrial fibrillation,” or an abnormal fluttering.
Third, consider threshold training, which are intervals during which your body becomes more anaerobic than aerobic. On the trail, do three to eight intervals of three to five minutes at a pace that feels comfortably hard (you can talk but not sing) with an easy, one-minute jog between each.