Mad, I think it can vary depending on the runner. I ran my P.R. [3:05] on less than 40mpw average but I was much younger  and much faster than when I averaged 55mpw for 12 weeks with a peak of 65 miles. Now I’m planning on using the Pfitz 18 week / 70 mile schedule in hopes of running 3:35 at age 48. I know guys that never run more than 15 milers and can run 3:12, whereas I ran 26 milers for the same finish time. Regardless, I would trust in the training you’ve done and after the marathon access how you could improve on your next attempt. Don’t get too concerned on what you could / should have done differently this time. Best of luck!