In general in this marathon training cycle, my avg HR has been in the low 150s on my runs, which is higher than a few years ago. But it hasn't been a start in the 140s and a slow climb into the 150s/160s. Instead, it gets into the 150s rather quickly and bounces around a lot. On an 18 mile run last Sunday, I pushed the last two miles (7:36, 7:35). Oddly, those miles were my lowest HR (low 140s).
@SteelCurtain picked up on that in Strava. Yesterday, up in Wisconsin, I did an 18 mile progression run. 5 miles easy (8:40ish), 5 miles push (8:15ish), 5 miles hard (7:40ish), then cool down. Once again, my lowest HR - in the 140s - was during the hard miles.

I'm puzzled. Maybe it's tied to my breathing pattern? For slow pacing, I've been using a 3/2 pattern. When I push, I switch to a traditional 2/2 pattern.