Now that a couple experienced marathoners have spoken up, I’ll go ahead and chime in. From your report, it sounds like you just weren’t fit enough to complete the race at your pre-planned pace. You say you felt like you weren’t going to be able to hold 2:40 pace almost from the start, and it sounds like you felt the crash coming well before it actually happened.
So here’s what I’m thinking. Compare your 10K, half marathon, and marathon PRs and a pattern emerges. For convenience, I’ll use Jack Daniels’ VDOT table to make my point, though the race conversion calculator on this site also works. Your 10K PR suggests about a 1:13 half marathon and a 2:33 marathon (VDOT about 65). Your half marathon PR suggests a 2:37:22 marathon (VDOT 62.5 or so). Yet your PR is 2:43 (VDOT 60). You tend to run significantly slower than predicted as the distance gets longer. So there’s a couple possibilities: 1) You’re not properly trained for the longer distances. 2) You’re built to race best in the 5K-10K range (or possibly even shorter), and that’s just the way it is. Let’s take a closer look at your training…
You seem to have plenty of background, run adequate volume, and run plenty (ALOT!) of long runs… all positives. But what really jumps out at me is that you seem to train at only 2 speeds… easy pace and your 3:20-3:30/km intervals. You seem to completely neglect tempo runs and marathon pace runs, which are generally considered crucial for success in the marathon. So as you suggest, the problem may be that you didn’t train specifically enough for the distance. I’m willing to bet you would improve in the future if you put more emphasis on tempo runs, regardless of the distance you’re training for, and maybe a little less emphasis on the intervals if training for a marathon. I’d be shocked if this didn’t make a significant difference, though again, there’s always the possibility that you’re more of a 5-10K specialist. Only time will tell.
While you had a disappointing race, it doesn’t sound like such a disaster to me. You found the strength to finish when you had nothing left in the tank, still managed to have a good time, and you’re striving to learn from the experience. It may turn out to be a great race after all.