Canova said on a recent podcast that a 30:00 10k runner should be able to run a 2:12 if they train right (140+ MPW for 10+ years). I don’t necessarily believe it but the point is you don’t need much speed if you train very specifically
When you run 30:00 for 10k and then 140 mpw for years to finally run 2:12 you will be a 29:30 10k runner at least.
Theoretically some people capable of running 2:20 or less with a mile PR of 4:30 or more exists for sure...the point is that A) they didn't make the 4:30 while they were in 2:20 shape but likely years before that and B) they never trained for the mile (or they did it many years before being even remotely close to 2:20 marathon), so if they did they would run of course faster than 4:30.
At the same time, the hypotesis of a 4:10 mile as "necessary" for a 2:20 marathon is ludicrous.
I don't know Patrick personally but I read a blog a while back that basically said he runs ~10 miles every day. He did 70-80 miles/week, staying healthy the whole time, for a decade. I'm sure he probably could've run a faster mile. But if you've run 4:10 or faster you should be thinking about something much faster than 2:20 imo. Anybody in the 4:20s or low-4:30s can try to develop the strength for sub-2:20. 4:10 & you're playing with different numbers.
I was the opposite. Ran 4:20 for a mile and 2:47 for the marathon (virtually perfect even halves on a flat course). I don't think there is anyone in the world that could have coached me to break 2:40.
Something doesn't really add up here. Let me coach you. Feels like exception/not the rule. Were you more of a low mileage 400/800 runner that popped off a 4:20 mile? Was the 2:47 later in life & well removed from the 4:20? 4:20 should get you 15-flat/31-flat. Not an easy jump to go from there to sub-2:20. No guarantee of that but definitely have a chance with good, consistent, mileage. I wouldn't believe you if you said the marathon was a year or two removed from the mile PB & you had spent that time training like a marathoner.