Running at max training effort (5.0), always

On the watch you set your max HR through settings...have you done this? You can also enter your resting HR...then the TE will be more accurate...

Maximum HR for an individual can vary widely (often by +/- 15 to 20 bpm) from the average values (calculated, for example, as 220 bpm minus age, in years). I know a lot of people with HRmax well into the 200s, and mine in my 30s/40s was around 165 to 168 (should have been 180-190 according to the formula).

If you want to make productive use of the HRM to help guide some of your training, you need to have a better idea of your actual HRmax (and maybe resting HR too, depending on the system you use). Google "maximum heart rate test, Letsrun" and you should find some good advice. In my case, I've found that in a good 5k race (not shorter or longer), I usually hit within a couple of beats (lower) in the finishing kick.

Simple answer is you haven't been doing aerobic training all your life. You've been running aerobic events using anaerobic energy. (Note that different people understand different things by anaerobic but you aren't working pure aerobic - you're using a lot of anaerobic glycolysis rather than fat-burning or aerobic glycolysis).

When I was your age, I can remember running a 1/2M in 1hr50 (8:30/mile avg) and chatting to the guy next to me and saying "My heart-rate is 177". It didn't feel difficult but I was never fast than that until I started running more very easy miles to improve my aerobic system. I did a 1/2M a couple of weeks ago where I was running at 7min/mile with HR around 160 and only hit 170 in the sprint finish.

Firstly it's about doing warmups and starting runs slow. You can't go out the door and immediately go up to your usual pace even though it feels easy. You do that and you kick in the anaerobic energy and high heart-rates from the beginning.

What I suggest you do is a couple of weeks running of up to an hour each day but don't let your HR go over 150-155. You might find that needs to be even slower than 6min/km but go with it - it shouldn't take long if you do it properly.

A couple of years ago I was coaching a guy who ran 1hr48 for the 1/2M and did all his 'easy' runs at 8:45/mile and his heart-rate would be in the 160s and he'd be gasping. He had to run at 10min/mile to keep under that 155 but he really started to enjoy his distance running. Within about two weeks I was saying "adjust that limit to 145" and a week or two later everything was being run at sub-140. I was amazed at how quickly it improved. I still got him doing one VO2max session per week where we didn't worry about HR. But the rest of his week was HR-limited and he made good progress.

It's also possible your hr monitor isn't working properly. Have you tried another monitor to verify it?

Wondering if OP is using a chest strap monitor or the wrist HR monitor (which are notoriously inaccurate). To train every day at a 5.0 TE sounds absurd already, and likely a failure in equipment and/or not properly set to tje proper physiological state of the OP