I figured I'd give a case-study of sorts from someone that some posters at LetsRun might call one of the "blimps" or "hobby joggers." Scroll to the very last 3 sentences for a summary.
Age: 30
Height: 5'5"
Weight: 155 lbs.
Weight at which I would have a NON-overweight BMI: 150 lbs.
History: I was overweight by about 30 lbs. from 3rd grade to 10th grade. In 10th grade, I chose to join the track team to work out with them. There was no 'making the team' because it was a sufficiently small school in a rural area, even though the team DID have talent and about 25-30 guys on it. In the fall of my 10th grade year (before running), I failed the gym-class mile with a 10:13. In the late spring of my 11th grade year I ran a 7:21 for the gym class mile. Then I started to realize that I could do so much more and set the goal for myself of going sub-6 before I left high school. I joined the cross-country team my 12th grade year, and finished the XC season with a mile PR of 6:45 and 5k PR of 23:08. Spring of my 12th grade year in track my mile PR's became 6:08 in competition, 6:06 in gym class. My weight at the time was 147 lbs., height 5'5".
I then ran on and off for 10 years.
In spring of 2012, I finally motivated myself enough to run multiple times per week without having someone to train/run with. For the last 2.5 years (up through today) I have run 4 to 6 times per week, doing 25 to 40 miles per week, depending on fitness, injuries, or whether I want to work on speed or endurance. Here are my 2014 season-bests (all timed on a track, unless otherwise noted), after 2.5 years of training:
100m: 17.22
200m: 35.50
400m: 1:20
800m: 2:53
mile: 6:05
2-mile: 13:22
5,000m: 21:14 (certified road-race)
4-mile: 28:41
10,000m: 46:32
Now compare to my PR's while leaving high school:
100m: 14.8
200m: 34-ish
400m: 1:17
mile: 6:06
3200m: 14:08
5k: 23:08
As you can see, I lost quite a bit of sprint speed, but gained quite a bit of endurance.
However, 3 years of training in high school never got me under 20 minutes in the 5k. 3 years of training as an adult never got me under 20 minutes in the 5k. I can't believe that 85% of Americans could ever go sub-16, even if they trained for 16 years straight.