I probably should have noted that I'm now 62, so the marathon was more than 25 years ago. I still do 5k road races, and masters track. I'd accept that I may not have perfected marathon training, but it certainly wasn't a question of not being able to push myself. If anything I over did that when I was younger.
What I was trying to convey is that even with a degree of athletic ability, your genetics are going to determine the distance range over which you can effectively deploy that ability.
My genetics predisposed me to the shorter end of things. I could do sessions of 3 x (4x150m) acceleration runs and crack 12:00 for the last 100m, and I ran 52:xx for 400m. While it's not exactly flying, for a club runner it indicates reasonable speed. The fast twitch that let me do that, combined with a decent V02 max, made me best at 3000m. I did manage to place in the U.S. Masters at 5000m also, but I never had a stage where I could actually run 5000m hard. I did also manage an 88% age graded 10k at 54, but generally always did get comparatively worse the further up from 3000m I went.
For me, however, build and percentage of fast-twitch, meant that I was always going to be limited at the marathon.
You are probably a runner with more slow-twitch, and than enables you to run much closer to your maximum pace - and you're clearly admirable determined - had I gone through 10k faster than my 10k best, you would have needed a dustpan and brush to clear up the mess.