I've been a chronic under performer at the marathon distance.
My marathon PR from when I was in my early 20's is in mid 2:30's. However, in the 3 months leading up to that race I ran a 1:09 half marathon, a 1:45 20 miler, and 15:00 5K. All of those suggest a marathon time closer to 2:20-25. I preformed similarly in a few other serious attempts at the 26.2 (mid to high 2:30s after ~1:10 halfs) both before and after my PR race with both more conservative and less conservative early pacing.
A few weeks ago I ran my first marathon in ~10 years in about 3:15 (I'm in my mid-30s now). In the months leading up to the race I ran a 17:45 5K, a 35:45 10K and a 1:21 half marathon. All these suggest a sub-3 hour marathon.
Back before my PR race I was averaging 80-90 miles per week and got up to a 2hr 30min long run. For my recent race I was averaging 45-55 miles per week and got up to a 2hr 15min long run. I'm 6 feet tall and weighed 145lbs and 160lbs, respectively during my PR and recent marathons. While I probably should have done more mileage and longer long runs, I don't feel like I'm way far enough off the mark on either to explain my under performance. Any ideas why someone might be so much worse at the marathon than shorter distances? Is my size an issue? A recurring theme is that I've fallen apart in the last 2-3 miles due to leg cramps, even with relatively conservative early pacing. I lose most of my time when I'm forced to stop and stretch out the various charlie horses that crop up in those last miles. Between stretching I'm normally able to hope right back on pace until the next round of cramps start. Any suggestions appreciated.