If he hasn't run a single mile since a very long period of times, his joints, etc. wouldn't be used to running that much, at an intense speed.
So I will say 20:10.
But after only 8-10 weeks of serious training, I think he could run sub 18min.
Sub 16 min after 6 months.
Sub 15 min after a year.
This seems very conservative. I was nothing special in high school, and as a scrawny 15-year-old maybe 6 weeks into my first season, I ran 3 miles XC in under 19. My teammates were like that too - after just a few weeks of training, it was pretty clear who was going to be decent, who was going to be good, and who wasn't. In one of the multitude of david45 threads, he asked how long it took people to break 20 in a 5K, and many of the responders said they did it easily in their first race. I'm sure a 25-year-old Bekele would have adapted to training much faster than any of us, assuming he didn't get injured.
Who was the Kenyan marathoner who basically did this - was an adult farmer when he started running competitively? Dennis Kimetto?
Exactly. Talent is everything