I ran approximately 10 miles today in 1:19:16, with an average HR of 138. I ran the same route as on Wednesday, except I turned around a tiny bit earlier, as on Wednesday I accidentally overshot my turn-around point. Comparing the two days, I ran the route about 2 minutes slower today, with an average HR that was 5 bpm higher. If I convert this to total heart beats over the course of the run, my total today was approximately 6% higher than on Wednesday.
Thinking to my new theory that dehydration causes increased HR, I recall that I was drinking quite a bit (and urinating quite a bit) for the first half of the day yesterday. In the evening however, I drank relatively little, and prior to running today (at 3 PM) I drank only a small amount of the smoothie that Kimia made this morning, drinking no other fluids for the entire day. So, today's run supports the increased HR with dehydration theory. I will try to drink lots of fluids this evening, and in the morning prior to running tomorrow to see if my HR decreases.