I ran 15 today during the hot part of the day. The shade-temperature was 74 (F) when I left, but the direct-sun temperature was 114 (F). When I got back the shade-temperature was 88. I couldn't measure the direct-sun temperature when I returned because my thermometer was no longer in the direct sun (I actually have two thermometers that share a common display in my kitchen. The outdoor thermometer is under my deck and is always in the shade. It has a wireless connection to my display in my kitchen next to a bay window on the east and south corner of the house). The kitchen unit picks up the direct morning sun.
I drank 40 oz of liquid (half Gatorade, actually the Costco Kirkham version, and half water). I could have used another 8 oz if I had had it. During the last mile, I only had a couple of sips at the start of the mile. During earlier miles, I took a big gulp of each liquid.
I'm still taking walking breaks at the end of each mile and drink my water and eat my Nature Valley Sweet & Salty Nut bar and my apple during those breaks. I also ate a banana before I left home. I took 3-4 extra walking breaks during long stretches (100 yards or so) of shade from rows of trees along the canal.
I really looked forward to the liquid each mile, and it was well worth the 2 pounds of weight (at the beginning) to have the liquid available. There were a couple of times when I had brief (1-2 seconds) spells of dizziness from the sun, and I was worried about heat-exhaustion. However, I felt ok and didn't get a headache, which is usually my first sign of heat-exhaustion, so I didn't worry about them. If the dizziness had continued, especially if the length of it or the intensity increased, I would have become concerned. Because I run the canal, there are fences between me and the houses I pass, and it would be difficult to attract attention from someone who could get me extra water. However, there are major streets every mile or so, and I could detour to a house if I needed to. I watched the ducks and ducklings swimming in the canal, and I was envious of them. I would have liked to jump in the water, but the canal water isn't very clean.
Even though my wakeup HR was 49, I got really tired and walked the last two miles -- I had no energy left for running. I tried to run a bit and found that all I could do was a faster walk, so I just kept on walking. I felt fine up to mile 9. At that point I could tell my calves were starting to tighten up. My energy level declined, and by mile 13, I was like a car out of gas :) I don't think I was fully recovered from my ordeal on Tuesday, and I'm sure the heat drained energy from me. Also, on the way out I had a 7-10 mph head wind (as usual, the wind died down before I turned around, so I didn't get the advantage of a tail-wind).
Still, it was a nice run, even if a "just made it" run.