A question for experienced long-distance runners

In My Humble Opinion
1

[There may be a factual answer to this question, though I place it here in IMHO since I’m soliciting expert opinions. Mods, please feel free to move if it would be more appropriate elsewhere.]

Background: I recently ran a marathon in 3:49:43, i.e., at an 8:46 min/mile pace. I did this by starting at a more conservative pace, and then increasing my speed in the second half since I felt I could, with the result that my first half was run at a 9:05 min/mile pace (total time 1:59:00) and the second half at an 8:27 min/mile pace (total time 1:50:43).*

My question: I’ve heard that the most efficient way to run a marathon is at an even pace throughout. Assuming this is true, is there any way to figure out approximately how much better my time could have been if I had run at an even pace? For example, should I have tried for an even 8:30 pace? Or would that have been too ambitious/not ambitious enough?

*Some finer-grained data, for anyone who cares, in case it makes a difference:
5K: 8:55 min/mile
10K: 9:21 min/mile
15K: 9:02 min/mile
20K: 9:08 min/mile
HALF: 8:37 min/mile
25K: 9:10 min/mile
30K: 8:37 min/mile
35K: 8:08 min/mile
40K: 8:16 min/mile
Finish: 7:58 min/mile

2

I will say you could have started a bit faster. How much faster is why marathon running is so tricky.
How did you feel at the finish?
Calculator. You can use previous race performances to predict race times and set optimal training paces.

Your current fitness, recent long runs, race day weather and so on can really be a factor in repeating a performance at the distance. Don’t forget, even a world-class runner can be forced to a halt by a very small pacing error.

Even pacing/running the second half slightly faster is generally best for any distance though running in a crowd will complicate that.

it appears that 5 sec. per mile on either side of even pace is the biggest margin tolerable.
Ex: 2 mile goal time of 12:00 will require the runner to come through in 5:55-6:05 for the first mile and still be capable of meeting the time goal.

The difficulty of marathon (or any long race of over an hour) is that the initial pace feels so easy that an early fast pace still feels easy and the detrimental effects don’t become evident until late in the race when the damage is done.