One other thought on this subject.
Besides getting the constant feedback that an HRM will give you this information will, in turn, allow you to know within a minute of what you can expect to run for the marathon. Come race day, given decent conditions, you should know almost exactly what pace you will be capable of.
Wouldn’t it be nice to know this about your NEXT marathon rather than having to attempt two or three more marathons and have them all be unknowns?
I predicted my last marathon within 6 seconds! And when I toe the line at Chicago in Oct. I will predict on this forum what I will run within one minute. If the conditions are right and I don’t hit it, I will buy all those present a round of beers. Promise.