Running economy
How much oxygen, and so energy, you use to run at a given pace.
Running economy is how much oxygen you use to run at a given speed. Two runners with identical VO2max can perform very differently if one needs less oxygen to hold the same pace, which is why economy is often the variable that separates athletes at the top of a field.
It is the endurance equivalent of fuel consumption rather than engine size. VO2max sets the ceiling; economy determines how much of that ceiling a given pace consumes; the fraction of maximum you can sustain determines how long you hold it.
It improves through several routes at once. Accumulated running volume over years is the largest, as the nervous system refines the movement pattern and tendons become better springs. Body mass matters, since carrying less costs less. And, counter-intuitively for many runners, strength training improves it.
That last point is well supported and still resisted. Heavy resistance training and plyometrics improve running economy without adding meaningful bulk, largely by improving tendon stiffness and neuromuscular efficiency, and the concurrent training literature finds endurance performance is not harmed by lifting.
Technique changes are less reliable than they look. Deliberately altering cadence or foot strike often makes economy worse in the short term, because you are replacing a pattern your body has optimised with an unfamiliar one, and any benefit takes months to appear if it appears at all.
Equipment has a genuine effect. Modern racing shoes with carbon plates and resilient foams improve economy by a few percent, which is large enough that governing bodies have written rules about them.
In practice, run consistently for years, lift twice a week, and leave your natural gait alone unless there is an injury reason to change it.