I'm just curious about what is the bare minimum 10k pr you must be able to run. In order to run a sub three marathon

probably a bit under 40. like 38:30 ish. not very quick.

According race calculators, a sub 3 is equivalent to somewhere around a 39:05-39:10 10K.

I think if someone runs relatively high mileage with minimal 10K or faster work, it's possible to be a 40-41 minute runner and break 3 hours.

Do six months of gradually building mileage, so you have two full months at 70 per week. With your 10k time and consistently careful buildup to avoid injury, sub 2:45 by the end of 2023 is realistic. Good luck.

My 'youthful' PBs (in my mid 30s) were almost exactly the same as yours. My marathon PB was 2:47:XX and still managed under 3:00 up until I was 41. (Then I took a break from marathons for 20 years)

To answer your question directly however, I would suggest a 10K of 38:40 would do it if you were well trained for the marathon distance.

And how long of a training block? 12-16 weeks should be plenty of time.

My 'youthful' PBs (in my mid 30s) were almost exactly the same as yours. My marathon PB was 2:47:XX and still managed under 3:00 up until I was 41. (Then I took a break from marathons for 20 years)

To answer your question directly however, I would suggest a 10K of 38:40 would do it if you were well trained for the marathon distance.

And how long of a training block? 12-16 weeks should be plenty of time.

i ran a string of barely sub 3s in my mid 40s and at the time would usually run 38:30 - 38:45 10ks. was always better at longer distances, all things equal, but even so, was not doing true marathon distance mileage (45 - 50 mpw) or many LRs (three or four in the 17 - 18 mile range). regularly did tempos and some speed work, though.

when i ran my PB of 2:56 @ 46 i ran a 37:25 10k, which scales about the same. same kind of training, but had good consistency over two years.

wasn't running when i was younger, but probably would have had better short distance speed relative to the marathon.

and, if i had trained properly for the marathon, i probably would have run a faster marathon relative to my 10k (or, said differently, a slower 10k relative to my marathon).

i think personal characteristics play into it, but probably also your training and age.

Similar to the guy above, I ran a couple of barely sub-3 marathons in my late 30s/early 40s. My 10k times were usually in the 38:40-39:00 range then. I ran in the 45-55mpw range training, with my longest runs about 16mi.

I am 20 years old and run about 40-55 weeks on average. The most I have have ever ran was 15 miles.

The more marathon training you do the faster your 10k will be, sorta.

Some say weekly mileage is more important than length of long run of the week. Maybe good advice for a 50 minute 10K runner trying to improve 10K. For a 34:xx 10K runner, length of weekly long run is important. Eg. if you have been able to race sub-35 10K with long run of the week 10 to 12 miles, that is not long enough for a sub-3 Marathon.

Bare minimum: sub-40:00 with great strength. For most: closer to 39:00 or just under.

Your PRs are much closer for someone looking at sub-2:40 or sub-2:45.

On that, you could probably run sub-3 any time you’re reasonably well rested right away

How long would it take you to get into marathon shape and run sub-2:45? Might take an 18-week build, might take 2 cycles spread over 8-10 months.

But I don’t know why you’d be very interested in the marathon now. If you want to up mileage, consider just doing so gradually and race a nice half. Do what makes you happy, but I think you could get to sub-16, 33:20, 1:15 with some fairly modest mileage increases — and see how your body responds to that.