Of course genetics is the number one factor, becuase there is nothing we can do about it.
Take a look at your average college team. They all do basically the same workouts, eat basically the same, live basically the same lifestyle, yet there is usually a big difference between the first runner and 7th runner. The variable? Genetics. Sure, the first runner may have a bit more drive, etc, but in my opinion it doesn't explain the gaping differences in time.
I realized this my senior year in college, and not coincidentally, this was when I realised I wanted to be done running seriously. I was training my ass off all summer and fall over the last 4 years, running 80-90 mile weeks, could break 1:50 in the 800 but not even come close to breaking a 4 minute mile and barely dipped under 25 in cross. How was it that a guy like Alan Webb could run 3:53 as a high schooler? I gurantee you the training wasn't better or harder. His diet wasn't better. I really don't think he was able to withstand more pain than me (I had literally run myself into the ground some races). He's simply much more talented than I am.
You need training to maximize your genetics, but natural ability is what determines your limits.