I have been reading these posts about heart rate and heat. I know it gets hot out there and the heat and the humidity can kill you down here. I ran my last brick at a nice and cool 89 degrees. It was 6pm and the sun was behind some clouds so it was really nice. Oh the humidity was 80% which made the heat index closer to 97 degrees F.
A month ago my brick killed me. My heart rate Lactate Threshold is at 160 bpm. I had to take walk breaks to keep my heart rate under 165. I was running at 10:45 mpm pace and as soon as I started, bang back up to 168 and then another walk break . I could only run for about two miles at the beginning at a 9 minute per mile pace before I overheated and my heart rate spiked.
So what did I do and what has improved my times? First let me tell you I biked 18 miles then ran 5 on Sunday and my run averaged 9:15 mpm. A month ago I would have been reduced to a 11 mpm slog within two miles.
I have always wanted to go faster, but I have not been able to do well in speed workouts since I have such a strong reaction to the heat. Well I am very simply doing treadmill speedwork in the nice and cool gym. I am able to run 7:30 to 8:30 intervals while keeping my heart rates in check. When I slow down for my rest interval my heart rate drops down back into zone 2 and then I crank it back up during the interval to 160 or so bpm.
Literally in a month I am able to go out in the heat and my times have improved by a ton. I am thinking it may be possible for me to finally run two mile tempo runs at 8 min per mile in the heat. Maybe within the next month.
Anyway thats my story and due to the heat and humidity, Im literally sticking to it.
Just try it out. Go out one day @ 3pm and run a 5K. See what your time and HR data is. Then run a 5k on the treadmill at the pace you want and see the difference.
I personally do workouts on the treadmill just because I can control my speed better for interval and stick to the pace and HR much closer but lately I have been running much more outside in the afternoons when is still pretty hot and humid. Nothing prepares you for the heat like the heat. Thats my story and I am sticking to it but again that is just my opinion and I dont know much 
On race day, the run is outside!!!
Peace
Maybe you missed the point. I am able to do speedwork on the treadmill once a week which has improved my other outdoor runs. The fact that I am able to get the neuromuscular workout that I couldnt get otherwise has allowed outdoor runs to be much faster.
Still running most of my runs outside with the exception of hard Tempo runs that require me to work inside.
Do you have a race coming which will take place under that kind of humidity and heat?
Otherwise I think it is unhealthy and might be detrimental to your fitness overall.I do all my running indoors not only because if the heat but also the high ozone levels that we have in NC.
In Miami… Any race between May and October most likely will be under those conditions unless you are pretty lucky with the weather. The longer the race the more you have to run in hot/humid conditions, if you are not used to it, its a surprise.
Peace
Miami Man half Iron distance Triathlon is going to be November 11th.
If the sun is out, the temps will be in the 90s with an even higher Heat Index. I can’t count on it being cloudy or cool so I am doing my long runs outside when its hot. I try to avoid the direct sun as it is a real killer as we all know.
The bad news is the half marathon will be in the heat of the day so that will be tough. However, if I am trained properly, I will be able to get my goal time of sub 2:00. The weather will be no different than those running at Kona in October. Hot and Humid.
The last 2 years the run has been very sunny and hot, and there are a lot of parts with no shade whatsoever specially around the dirt sections.
Peace