There are few good physiological predictors of running times, the best being maximal oxygen uptake (VO2max), which accounts for about 75% of 5K running times at the world class level and almost half of the average runner's time. The average healthy young non-running athlete might have a VO2max of 35, which suggests that with 6-8 weeks of training, they could run a 5K at about 9 minutes per mile. A year of running and occasional racing and that VO2max might rise to 45 and lead to a 5K at 7.5 minutes per mile. Years of hard dedicated training might end with 55 and 6 minutes per mile. If this runner is a tailor-made 5K runner, who just happens to have a low VO2max, that 5K time might get just under 18 minutes. That's the limit - any time less than that requires above average inborn talent (genetics).
The current world record for the 5K is under 13 minutes. That 5 minute difference is completely talent-related. A 13 minute 5K runner couldn't achieve that time without serious dedicated training, but an average runner could not attain it, no matter how hard they trained. Each competitive runner has to make the decision of just how hard they're going to train, knowing that they might train just as hard or harder than those they will never beat. The good news is that you're probably a lot more talented than you think; the bad news is you're also probably a lot lazier than you want to believe.
You can't ignore inborn ability. Peter Coe, father of world-record 800m runner Sebastian Coe, wrote (with David Martin) a book on training middle-distance runners. In it, it's suggested that if one follows the principles they espouse, one can run like Seb did. They gloss over the fact that Seb at age 13, having run a few miles 4-5 times a week for a year, ran the world record 800 for his age; he simply started faster than everyone else in the world. They also gloss over the fact that these principles were developed after the fact and that Seb's coach Fred Gandy didn't follow them.
When I was 20, I ran a 10K that also illustrates this. I had a bad race, running (I think) 35 minutes, finishing about 12th of maybe 250 runners. Finishing second or third, running 31 minutes, was a guy who just found out about the race because it was starting in front of his house. He was a high school record holder and a good, though not great college runner a few years earlier. I happened to know his sister (we had a class together), so I asked her how he trained. "Well," she said, "he cuts the grass once a week. That's about all the exercise he gets." He had experience and his body had been developed by years of running, but a two year lay-off meant he was running with no training; yet he ran faster than I ever have. Talent beats training almost every time - you learn to live for that "almost."
There is a flip side to the story, of course. Derek Clayton and Grete Waitz both had VO2max numbers of about 70 (I seem to recall 69.7 for him, 71.6 for her, but I don't know where I'd check that), which suggest 5K times of 14.5-15 minutes, which was world-class for women but not for men and which explains why she ran 5K and 10K races on the track and he didn't; comparable marathon times would be about 2:20. Waitz ran her first marathon on 70 miles per week, with a long run of 13 miles... and she set the world record at 2:35. Though she didn't especially care for the distance, she was able to set a record without specific training, so she started training for the marathon and whittled her time down to 2:25 over the next few years. Clayton lowered the men's world record 15 years earlier from 2:12 to 2:08, training at least 140 miles per week, usually 170, and sometimes 200; he did much of that mileage at 5.5 minutes per mile or faster on hilly terrain. According to his VO2max, he shouldn't have been able to run that fast, but VO2max only accounts for about 10% of marathoning ability. He was not the most physically gifted of runners in all respects, but he made up for it by training hard.
In his book, Brad Hudson points out that the current women's world record-holder in the marathon (Paula Radcliffe, 2:15) ran 150 miles per week and says that her record will never be broken by anyone running substantially less than that. History suggests otherwise. Steve Jones broke Clayton's record running only 90 miles per week - he had more talent and didn't have to work as hard.
You can't do anything about talent. You can do something about how hard you train. That's why you focus on training.
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