I have a similar background - did my first sprint tri in 2014 and was predominantly tri based from 2014-2018...2019 has been more running. Running years have been
65, 500, 800, 1200, 1200--this year on pace for 2,000.
the main difference between us is that I started short and stayed short (have done sprints, Olympics and one HIM) - you went long.
Likely, you got into the routine of "slogging out" a lot of runs to get the distance in vs. I did a lot more shorter stuff. My HM PR is 1:22 (first HM in 2015 was a 1:40) with just "running" - no workouts...and now am prepping for my first marathon and started actually studying how people train - indicators suggesting low 2:50s at this point.
my point is - I think that to get better at long, you should go short - skip the marathon in november and do a half - try to get your half under 1:25 and then 1:20 over the next year or so - you seem athletic, have good build / weight. Once you get that down, revisit the marathon and go for a 2:50.
marathons are draining to train for and race (i've been told) - half marathons are not. Get fast (by local standards) - get some ego boost, and then go back to the marathon.
Skip the marathon. Forget about it for 18 months.