In general,however, research studies support that running economy improves with higher volume, slower running. So longer and slower distance training is more effective as a means to improve economy. The reasons for this include the increase in mitochondria, which means more effective use of oxygen by muscle. Also, it's been found that the longer and slower running eventually leads to a 'learned' neuromuscular response where the vertical oscillation of the runner is reduced. In otherwords, less time going up and down, more energy saved, and this is simply a function of repetition!
Now here's where things get tricky! Many of you are probably thinking "what about speed work?" Surely that will see massive improvements in running economy? There's always confusion about whether faster runners are more or less economical. And here, the general rule is that it follows what one might call The Law of Specificity, which basically says that you'll be good at what you train for! In otherwords, if you are a middle distance runner (800/1500m), then you'll be more economical at higher speeds than a marathon runner at those higher speeds. The interesting thing is that it's been found that this same middle distance runner then becomes less economical at the slower speeds than the marathon runner. So again, economy is good where you train it, which to me really re-inforces the value of training specifically, and how important co-ordination and motor control are! -Dr. Ross Tucker and Dr. Jonathon Dugas, The Science of Sport