Recent HM in October of 2:01:10, which was a PB. I guess that projects to a 4:10 Marathon? Past 10 years I have only done HM, 10k and 5k. Only one marathon, under trained, as a beginner runner 12 years ago.
The past year we have been very consistent. Over 45 if that helps with the decision.
Also, I am currently doing your maintenance program based on HR. I was thinking of switching to pace based for the marathon plan and then on long runs switch to HR?
If you are having a tough time deciding between levels my usual response is to start on the lower level, give it 4-6 weeks and level up if feeling strong, nailing or your sessions, and have time for more training. Train where you are, not where you want or think you should be.