In writing this I'm just replaying my early younger days of running so hopefully some of this might help. My hope in replaying this it is useful. I know that you've been running for quite awhile so you probably have learned this stuff abouty efficient pacing and heat etc. pretty well by now.
This all makes me recall my jogging days back to when I a young pup of 40-45 yrs of age as a very slow-but-steady-runner. When I first started out with longer and longer distances fun-runs, I learned to suppress the desire to start out fast....especially in the hot weather.
My engine many times would shut right down over 80 deg temps and also with Boston's high humidity sometimes regardless of how much I drank. For those longer runs, I ate my bananas and Fig Newtons but learned that the diet the night before was also pretty important to avoid cramps and running out of gas.
I learned to never add any new foods. I learned the hard way about trying something new like Gatoraid during a race. I never tried anything during a race that I wasn't used to. I was better off with plain water.
I spent many weeks training for the fun-runs learning what pace was good for me and sticking to it. It turned out that when I ran more than 2 miles, the most efficient pace was around 7.5 to 8 minute per mile. However, when running for more than 30 minutes, it moved down to around 8:30 per mile. The good thing about those organized fun-runs...as you get the mile markers you can figure out where your pace was from the splits.
Key for me in the fun-run was focusing for the first mile pace at the pre-selected time (say 8:20 for the first mile) regardless. When I did this right, being as consistant as possible at each mile marker would often gain me 5 - 10 minutes when I ran a 10k race.
When the start line was a little crowded I would move out to get out of all the traffic but then afteward I'd settle down after the first quarter mile. If I had any gas left in the tank, I'd pick up the pace about 10% in the half mile or the last mile.
Well it was fun remembering.