Just one component is not the key but rather all 4 are key.
Consistency, Capacity, Frequency, Mixture
In order to make the most gains and realize your full potential as a runner you have to maximize each area in unison with the others.
What does that mean?
That means get in the right mixture of work: speed work, threshold work, long runs, races, and don't let any area fall behind. You have to work on all aspects year round.
Do this consistently, week after week, month after month and year after year. There is no short cuts in running, it takes time to realize your full potential.
You need to improve your frequency, slowly over time until you are running approx. 13 times per week.
You need to slowly build your capacity/mileage over time, but only in conjuction with the proper mixture.
Big jumps in either mileage or quality don't work 9 out of 10 times because they aren't sustainable.
No short-cuts, just look at these 4 tenets and slowly and methodically over time improve on them. Maintain good mixture while increasing your capacity and frequency.
Too many people just pick out 1 of the 4 areas and then go overboard on it and that just doesn't work in the end.
Runners are like chains, we are only as good as our leakest links.
Improve your consistency until you are training solidly year round except for occassional planned short regeneration breaks.
Improve your frequency until it reaches 13 times per week.
Improve your capacity until you have reached your own personal maximum amount that you can do wwhile keeping up the other areas.
Keep you mixture in balance with regular work on all areas.
You won't be able to make big changes to any area very quickly but will have to work on all areas (starting with whatever your weakest is) slowly over time.
Too many kids in high school or college (and many post collegiates as well) want to jump to extremes, dramatically jump their mileage or frequency or even their quality work. That is short sighted and just doesn't workout in the end because it's not sustainable. Instead start today to work towards the only true long term solution, that is to work on slowly improving the 4 components of training over time.