A few points, IMHO:
1. For the majority of people especially those who post here (me included!), genetics plays a minor part.
2. Childhood and adolescent lifestyle also plays a minor part. Loads of former couch potatoes here (me included!).
3. The major areas that can lead to fast running is:
- training approach: sensible schedules, nutrition, rest, etc
- realistic objectives, personal commitment, consistency, positive attitude, discipline etc
- running style & technique in terms of a skill which anyone can learn.
The part on skill is hardly ever mentioned.