My wakeup HR was 47, and I ran 5 miles on the Jordan River Parkway. I was late getting out, and the temperature was in the low 80s (F). Most of the run was in the direct sun, and I drank 24 oz of liquid (my normal ratio of half water and half Gatorade). Because I was well hydrated, I didn't mind the heat at all, even though the sun burns more than it did when I was in Massachusetts. The difference is that here I'm at an elevation of 4800 feet while in Massachusetts I was at a typical elevation of 300 feet.
When I run the Great Salt Lake Half Marathon next week, I will be at an elevation of 4200 feet. I don't know if going down to that "lower" elevation will give me much additional oxygen, but I "hope" it does help a bit. After I had been in Utah for about four years, I went to Connecticut for my daughter's graduation from college, and I found it much easier to run there than it was here in the Great Basin. Getting sufficient oxygen is the name of the game in running long slow distance.