I've recently incorporated 80:20 training into my routine and try to keep within heart rate (reserve) zone 2 for a lot of my runs. I've noticed that often my pace improves throughout the run, getting progressively faster whilst maintaining the same heart rate. Here is a particularly clear example, where my first km was 6:52/km pace, but the last was 6:07/km. On these runs, I am generally trying to run as fast as I can whilst keeping to zone 2 (I occasionally drift briefly in to the bottom of zone 3) as that is more enjoyable. I don't perceive an increase in effort through the run. I was using a chest strap heart monitor, so I think the data is fairly reliable.

I don't think this is particularly problematic, just curious as my intuition would be that my pace would drop as I get more tired?

Thought I'd add some running dynamics data in case that was useful:

It looks like my stride length is increasing, as the vertical ratio (the ratio of upward and forward motion per stride) decreases, which suggests I am running more efficiently, but that is not saying much as I can't run faster with a constant heart rate without running more efficiently. My cadence is also increasing, but only very slightly.