Hi,
In Spain, a coach named Rodrigo Gavela developed a test to be run 10 to 14 days before a marathon. First you run 6000 metres at your marathon intended pace, then you rest 90 seconds and you run another 6000 metres at full speed. If you run 21:30 on the first 6k you need to run 19:36 on the second 6k and that gives you a best possible time of 2:31:12 in the full marathon and a more likely time of 2h33m. He has the tables in the link. Its not scientific but it kind of works quite nicely if you have enough volume trained, etc. It worked very well for my 3h03m marathon last year (25:30 first 6k and 22:48 second 6k).
Anyway good luck if you try this.
https://www.foroatletismo.com/...vela-maraton-2x6000/
Thanks for posting. He says it may be most accurate for slightly slower runners, but still could be worth a try.