Yeah these comments and bang on - I researched / talked to some running friends and was not going to be effective to continue running at 4:30-4:40 if I wanted to build endurance and form to run a marathon.

The variation in runs is also correct - that was part of the mara training program. I guess what I found interesting was that my interval/fast runs in late 2022 were at a lower HR than late 2021, meaning at a rudimentary level my fitness had improved.