Thats not what you originally implied, but lets not argue.
I don't suggest for a moment that all 3.33 men can run fast marathons. What I'm saying is guys like Tergat, Geb, and even KK have much greater potential to run a fast marathon than a 3.49, 28.02 man like Deek IF they can develop the endurance capacity.
Clarke ran 13.16 and 27.39 on shit tracks but couldn't break 2.20 for the marathon or 3.44 for 1500 (on a shit track). Geb and Tergat would've thrashed him in a 1500 and also in a marathon. It stands to reason that their superiority at 1500 would extend thru 5 and 10k. If they are 10+ sec faster at 1500 and 14+ min faster at the marathon then clearly 30-40s at 5 and 1.00-20 at 10 are predictable improvements.
I dont think you can do the extrapolation/interpolation from the 1500m/marathon into the 5k/10k range for an individual athlete. In the example given, Clarke was relatively untested at marathon and never focused on it to the best of my knowledge. Tergat and Gebr have both peaked for the marathon. Its not a fair comparison.
Rod Dixon was a 3'33/2'08 runner (in NY aswell). Where does that put him over 5k/10k? Well better than his PRs, thats for sure.
I'm not arguing that there is no correlation between 1500/thon and 5k/10k. Of course there is BUT for individuals who focused on different events at different time to different degress its not a robust comparison.