Note: This doesn't seem to be a warm-up issue, see the updates at the bottom.
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.
The only answer so far cannot be right. Here is my 15k run this morning (this time on the track). Again HR recorded by chest strap, so should be reliable. My pace improved from 7:38/km at the start to 6:43/km at the end. I really can't be still warming up after running/jogging for an hour and 45 minutes! (I had gels at 5k and 10k, which explains the lack of improvement in pace at those points - I lack the coordination to deal with the gels without it affecting my running - it takes me most of that km to consume them.)
Update on the Update:
I ran a 10k in zone 2 this morning without a warm-up (was short of time - not something I generally do), and this time I started out faster, slowed down a bit and then sped up again in the last 4k.
If the usual gradual speed up was because of not being sufficiently warmed up, running without a warm up ought to exacerbate the gradual increase in pace, rather than slowing down in the first few km. Only one run, of course.