I think pace may also have something to do with that. My best 10k was 32.5mins and that pace would give a marathon time about 2:17, whereas my best was 2:38. Like you say, knackered after both but then the pace was so much faster for the 10k.
I found Marathon Training gave me great pace judgement.
They're excellent times, chapeau👍
I was never a fast runner but I had good endurance, and like you the Marathon training and track running gave me good pace judgement. ( I could never get under the hour in a ten mile race but I could go through 10 in about 58 minutes in a half marathon 🤔) I remember doing the Reading Half in the early 90's, having a good run and as I overtook a usually much faster club mate he turned and said "What the F*ck are you doing up here" That gave me the motivation to push on and drop him 😁