(1) The average hr for the first mile is low compared to the others since the heart jumps from 100 bpm to 175 bpm in the first mile
(2) Even though pace and effort were maintained throughout the first five miles, heartrate slowly increased. A quick Google searched revealed the increase in heartrate may be attributed to Cardiac Drift. During long and strenuous workouts, the heart has a tendency to beat slightly faster even though you do not feel as if you are working any harder—this is called cardiac drift, or cardiac creep. The heart rate increases even for the same effort expended because as you sweat, your blood volume decreases due to fluid loss and also your heart tries to do its part in regulating body temperature.
(3) At the 5 mile mark, a decision was made to pick up the pace just a bit to see where the heart rate would go. The graph shows an increase in the heart rate change right around the 5 mile mark (a slight upward bend in the slope)
(4) In the last mile of the race, heart rate peaked to 191! A slight downward bend for the last mile may reveal a tipping point, where heart rate begins to decrease (resulting in a slower pace) or levels out (also resulting in a slower pace). Using a heart rate tipping point of 187, taking cardiac drift into consideration, and assuming weather/course are very similar, a 5:41 pace should be maintainable for 13 miles (1:14:30 HM)