I'm a former 800 runner whose PRs have always been better on a comparative basis as I move down in distance. While I ran cross country and had periods where I hit 100 mpw, my longer distance PRs were weak in comparison to my 400/800 times. I suspect the 400 may have been my best event as I never trained specifically for it and yet my PR was equivalent to my 800 PR bases in the IAAF tables.
I am now later in life and have taken up the marathon. Similar to my past experience, my 5k to half marathon times indicate much better marathon potential than I have been able to achieve. My mileage has peaked at 60 so I'm not running a lot of miles for the marathon distance, that said, I have a huge gap between my marathon and half marathon PRs.
Is the answer simply a matter of running more miles? What strategies have people that naturally lack endurance used to have marathon success?