Alright, so we'll plan on a T+D of 132. Thankfully, it looks like there should be some decent cloud cover. Here's hoping. A T+D of 132 is a 2.5% adjustment.
In August you ran a HM in 2:51 in a T+D of 153. So adjusted that would have been a 2:43 HM under ideal conditions (which btw your May HM of 2:49 was ideal conditions and thus your August HM was a better performance). Under the same conditions as the Marathon, you're looking at a 2:47 HM.
That performance predicts:
6% chance of 5:48 marathon or better
10% chance of 5:50 marathon or better
25% chance of 5:58 marathon or better
50% chance of 6:10 marathon or better
You also ran a 33:49 5k at a T+D of 139. So that's like running a 33:39 under the same conditions as the Marathon. That equates to:
6% chance of 5:22 marathon or better
10% chance of 5:29 marathon or better
25% chance of 5:51 marathon or better
50% chance of 6:31 marathon or better
You see a bigger spread because a 5k is less indicative than a HM based on the distance comparison to a M. But I'd say the 5:51 and 5:58 guesses are in-line with each other. So I think we should aim for a 5:45-5:50 finish time (13:09-13:21).
We'll use this runner for split adjustment purposes:
https://www.strava.com/activities/1888115177/overview
One bonus, it looks like all three runners on Strava show that GPS measured about 25.8-26.0 miles, which gains us a few precious seconds.
These paces are very close to your adjusted LR pace for the conditions (13:19). So my suggestion is to treat the pacing the same. Use your same run/walk intervals, with the goal of staying in the pace window laid out above for each mile split. You can ABSOLUTELY go slower than these if you feel the need to, especially early on. Don't force it. But under absolutely no circumstances should you go faster than anything listed here until AT LEAST Mile 20.
This is a challenging course with tons of variability in the miles based on elevation changes. Miles 3, 4 and 22 are especially challenging. Since miles 3 and 4 occur so early in the race, they are of vital importance. Do NOT run these too fast as that will make the remainder of the race that much more difficult. Take them very easy and calmly. This course is dead even in terms of split pacing. The first half is projected 2:52:09-2:54:45 and the second half 2:52:10-2:54:46.
The race will feel harder towards the end. That is completely normal. I've done 15 marathons to date. And only once did it feel easier in Miles 17-20 than it did at other times in the race. So expect that it will be hard and tough and painful. And that the difficulty of the race will ramp up in the last 3-4-5-6 miles. But you've done those distances plenty of times in training. And you can do them again at the end of the marathon.
In 2019, two runners finished over 6 hours. In 2018, two runners finished over 6 hours. In 2017, six runners finished over 6 hours. Twice there have been people who finished in 7 hours. So don't let the 6 hour time limit play mind games with you. Run your race and finish at your pace. If that's over 6 hours, so be it. Doing anything more than what your body can do on that day, especially early on, will be destructive to your chances of finishing. You CAN do this. I know you can do hard things. And this will be hard. Mentally accept that going into it, and it will make it that much easier. You've birthed a child. Running a marathon is not as hard as birthing a child. Stay strong. Smile. Enjoy your first marathon. Something will inevitably go wrong during the race. Let it wash over you and do not dwell on it.
Think of all the challenges you went through just to get to the starting line. A race is not a final exam/test. A race is a celebration of all the training you've done to get there. It's the party at the end, and not an assessment of you as a person or as a runner. So many things can happen in a race. So enjoy your victory lap for completing the training!
Thoughts?
