You can try testing it on track, put your bottle at one corner cool yourself down from time to time
Yes, it's a different workout but it was harder, I was doing 3k easy, 5k steady, 6k HMP. I usually go for harder workout compared to the previous week, to see if I can improve on that. For that run, I tweaked my strategy and then run longer and faster.
I managed to run faster at a longer distance, not having to exert as hard. And I think I can still improve on that, since it's a work in progress.
First 3km: 5.20
Mid 5km: 4.55
Last 6.14km: 4.32
From a comparison standpoint, it's definitely not as exact as what you are getting at. But I want to keep tweaking the strategy to optimise it to be more efficient and also injury-free.
I am more concerned with heat and recovery runs. Not really on cardiac drift. The avg. HR is a lot higher during recovery runs because of the heat. I believe that slowing down is important but it shouldn't come at the expense of changing running form too much. Most of the times, I just go with vdot/macmillian calculator for recovery pace at the slower end of it. This would still give me 150+ avg. HR because of the heat. During the run, I could still run at conversational pace with my sibling. The national triathlete, Aaron Lee went from threadmill running with air con at 125 avg. HR to 153 avg. HR, despite slowing down his pace 5.27 > 6.07.
At the end of the day, what matters is that my legs are not feeling too fatigue after the easy run. And it serves that purpose even though I am running at 150 avg. HR, which isn't considered as easy by the watch. If this was the case and I have been doing this for weeks, I will be injured by now for going too hard continuously for my easy runs.