Training for running is very counterintuitive.
Greatly oversimplifying: Running fast builds strength and teaches your body to burn glycogen. Running slow builds endurance and teaches your body to burn fat. Running slow builds capillaries and mitochondria.
Racing distances less than about 400 meters requires speed. Racing distances over about 400 meters requires more endurance than speed.
If you run with a good (cheap ones are worthless) pulse oximeter, you will find that your blood oxygen stays about the same no matter how hard you run. Your blood will have the same oxygen level even when you are running as hard as possible, gasping for breath, and feeling like you are strangling for lack of air. That's because your lungs are fully doing their job of getting oxygen into your blood, and CO2 out. But your capillaries and mitochondria are not up to getting that oxygen into your muscles. And it's your muscles that are screaming for oxygen.
Running slow, defined as being able to speak full sentences without straining, builds capillaries and mitochondria. Running fast does not.
A good forum for running is:
www.runningahead.com/forums. You will find it worthwhile to surf there.