I think the people that can best answer the question are runners who really have no talent (or put another way, perhaps those right in the middle of the bell curve of talent).
I'll submit myself as a possible example. My 5K PR is 19:47, but I never trained specifically for it. So, I'll relate where I am now as a possible proxy.
At 56, I just ran 22:23 on a track on January 30. This was after a two-month break from running in the summer and starting again in August. Started doing workouts in mid-October (every fifth or sixth day) and running a little less than 50 miles per week.
Since I met my goal of 22:30 for 5K, I moved on to the 10K and am now training for a Half.
The most specific training I did for the 5K was 5 x .6mi (7:00 pace). I didn't do any kind of repetition pace training (so not sure how much that would help or what else might) but I feel as though I couldn't ever go much faster than the 22:23 I just did (maybe if I worked and worked at it I could maybe run 21:47, or 7:00 pace?).
As it is my 22:23 is about 18:00 age-graded, maybe 21:47 would be at best 17:30 I'd guess.
So, I'd submit somewhere between 17:30-18:00.
It's seems ridiculous to me to suggest runners with no talent can run 16:00...