I have no idea how to do base training in the heat

I live in the deep South, also. You're right, the heat and humidity are brutal. It's tough even if you already have an aerobic base, let alone if you're just starting out.

Things you can do:

Run at sunrise when the heat is at it's low (sunset is still very hot)

Allow yourself to go very slow, even taking walking breaks, if needed

Stash a drink bottle somewhere to hydrate during your runs (I put a bottle in my mailbox and do multiple loops, stop & hydrate)

Do more frequent, but shorter runs, to keep mileage same

Most of all, realize that you are gaining fitness (your heart rates prove it) and that if you keep it up, come late Sept or October, you're going to feel like a million bucks when the temp and humidity drops and be able to run faster, easier, long, with less effort.

Just hang on, you're not alone; we're all hating the summer-sufferfest. Just a bit longer and we're through it.

There are hundreds of thread about this. All you can do is struggle on.

My advice FWIW:

-don't look at your watch/pace. It's useless and mentally self defeating. Ignore it. When the weather breaks you will feel like a 50 lbs weight has been lifted off your back.

-Seek shade, seek trails. For me this does two things, one I don't really pay attention to pace on trail runs (see above), two it's usually a bit cooler in the woods.

-Run on a shorter loop course. Some days the temps are so bad that the body just can't deal and you have to bag it. If you can tackle a few laps of a 2-3 mile loop vs getting 6 miles from home and then having to struggle or walk back.

-Make it OK to bag a run or cut it short when things go sideways. Sometimes is so hot and your body just cant take the run. Maybe it's what you ate or didn't eat, or drank or whatever, but some days the body just says "NOPE!".. you need to be OK with that because you can really put yourself in a bad situation if you try to be "tough".

- Treadmill.. I know I know.. they suck. But you can do a sold 1 hr workout on TM and it's not too bad. I wouldn't want to slog through for hours and hours on a long run, but any time you can put in on the 'mill is that much less time you are dealing with the crippling heat.

Good luck.. it's a grind

Go slower than you think you should, also find shade. Think of your base building as "time on the feet" - meaning, the sheer fact that you are out doing it and doing it consistently is more important than pace per mile, etc.

Run slower. Drink more. Take walk breaks. Remember electrolytes. Ditch the watch.

Plenty of good tips so far

One more possibility:

If you have a pool, do loops from your house and loop back home every mile or so and do a quick dip in the pool between loops.

I live in Florida and used this approach on some summer 20 milers while training for fall marathons. While I was doing 6-7 mile loops, there’s no reason you couldn’t do the same thing for 1-2 mile loops.

You can also just use a hose to achieve a similar effect if you don’t have a pool or don’t want to mess with putting shoes and socks on and off repeatedly.

Also a Florida runner like Smoove. I've carried ice cubes in my hands and put them in my pockets to gain a mile or two or relief. Using the loop idea suggested, add grabbing handfuls of ice to your fluid stop (maybe a cooler full of cold drinks and ice).

Shouldn't you adjust your HR zone upwards if you're running with a high heat index?

I get that heat does place physiological stress on the body that is somewhat equivalent to running harder, but for things like muscle damage it's obviously not the same.

Presumably you will recover similarly from an easy 8 miler at 170 bpm in the heat vs a similarly paced one at 60 deg. Maybe not?

This is accurate. I normally do my Saturday 10-miler around 76-80 minutes but when I was in Naples, same effort, it was 83 minutes. I ran at 9 a.m. and that was still too warm (6/12/21). It was 85-90'F even then!

Where I live I embrace the summer season; the other 8 months mostly feature depressing, interminable rain. I'd rather be in your situation.

What I also do is two down-and-backs with water at my car.

Three Creeks in Pickerington is a great course for this. I do 6 out, 6 back, get water at mile 12 (by my car and I refill my handheld), then do 4 out and 4 back the other way to get my 20-miler in, even if it's hot.

https://www.metroparks.net/parks-and-trails/three-creeks/

I was a spectator at track meet over the weekend (NJ Meet of Champs). It was hot as b@lls until the clouds rolled in later in the afternoon. Being in the sun for a few hours, my resting HR at times was 30 beats higher than it usually is.

Moral of the story, it's ok for your HR to be higher running in the heat. Run by feel. You will need to run slower , but don't get too hung up on the numbers . The more you run in the heat, the more you'll adjust too, and you should start to see your HRscome down for the same relative effort.

I agree with those who say heart rate isn’t the best way to measure efforts. The heat. My heart rate in threshold pace workouts when fit is often in the high 160s to mid 170s.

My heart rate often gets up that high on an easy run in the Orlando summer, even though I can run at that heart rate for a 6-10 miler while conversing.

Clearly my heart rate isn’t telling the entire perceived effort story, so I primarily go by feel and let my heart rate do whatever it does.

Summer after my freshman year in college we had 40+ days of 90 degrees or more that summer. It was still roughly 85 degrees at night when the sun started going down, that's when I usually ran since i was working a job that started at 7am, I wasn't trying to wake up at 5 to go running. So I ran 70+ mile weeks starting my runs somewhere between 8pm-10pm.. I'd plant cold water bottles along my route so that I could pour it on myself when I reached them. It worked, I survived. Not much else you can do. Heat training is almost like altitude training. Your body makes some adaptations that actually benefit you.

Don't run slower all the time. Running at what would be your proper pace and taking walk breaks is better from a mechanics point of view. Use alarms for stop and start heart rate.