another thing to be wary of with following an "off the shelf" plan is sticking rigidly to the mileage of every run. Obviously, a marathon is 26.2 miles for everyone, whether you're completing it in 3 hours or 4 or 5. But taking, for example two runners, who both decided to follow a 55mpw plan, because that's what their schedule permits. One of them is aiming for 4hours race time, one for 3hours. So already you see a 25% difference in their paces. Because the schedule prescribes specific distances, rather than times, the slower runner, across all of their runs, will average 25% longer on his feet than the faster runner. A 40min recovery run for one of them is a 50min run for the other. A 20mile run will take just over 2 and a half hours for the quicker runner, but the slower runner will still be running 40 mins later.
I think this is worth bearing in mind no matter what your race target. Listen to your body. If the plan calls for a recovery run, it shoud be just that. If your recovery jogs are 12:00/mile, decide whether running for an hour will really aid recovery.
i'll be honest, one of the trickiest things I find with marathon training plans is distinguishing the different training paces.
run X should be 10-20% slower than mara pace
run Y shoudl be 15-25% slower than mara pace.
if i try to run 10% slower than MP, i could be anywhere between 5% and 15% depending how knackered I am.
Even more confusing is trying to reconcile paces with HR. So a certain run should be x% slower than MP, which equates to Y% of your maxHR. But whenever i've bothered to wear my HR monitor I find that my HR goes from resting to ~65% within a couple of minutes of jogging, then up around 90%+ whether i'm doing tempo, speed, intervals, MP segments. I suppose either I have a heart condition, or my heart lacks the nuance required to train at specific HRs