The methods behind distance running have evolved greatly over the years, from the long hard miles endured by runners in the early part of the 20th century to the Finnish introduction of hard interval work. HIIT (High Intensity Interval Training) has become a popular training style due to both its efficiency with time and suitable acronym. Tabata intervals are an even more truncated version of interval training, proven by scientific research (one must always be careful when this is your argument).
Nevertheless, the long, slow, base training has come back to the forefront, as we are looking again at our aerobic capacity and its potential. The 80:20 rule for low:high intensity ratios is now used as a suitable guide to organize training.
However, this leaves us divided between two schools of thought. One side focuses on hitting key workouts, eliminating junk miles, improving efficiency and training at high inensities for periods of time. Authors include Brian MacKenzie (Power Speed Endurance), Bill Pierce, Scott Murr, and Ray Moss (Run Less, Run Faster), and Kelly Starrett and TJ Murphy (Ready to Run). The slow-twitch junkies are equally reputable with Mark Allen (The Big Book of Endurance Training), Matt Fitzgerald (80/20: Run Stronger and Race Faster by Training Slower), and Phil Maffetone (The Maffetone Method).
It is easy to argue for either side of the training spectrum, as speed and efficiency are just as integral to running as aerobic capacity and musculo-skeletal adaptations to volume. As frustrating as it is, the answer to the question of which is more important is the one we hate to hear - "It depends". The dependency is on where you're at in terms of history, injuries and what energy systems you're skewed towards. Thus, this discussion will span a few posts to allow for the different methodologies to be scrutinized.
My interpretation is the following: Learn how to run fast, then learn how to train at slower paces. The title of the blog post has been taken from Daniel Kahneman's "Thinking, Fast and Slow". Similar to our brain's capacity to engage in fast and slow processing, our bodies have the ability to function at different paces, all of which must be trained to have balance.
Here's distance running great Haile Gebreselassie and American sprint record holder Tyson Gay in an Adidas commercial. Haile has kicked and closed many of his 10km races on the track at very fast speeds. You don't have to lose all speed to be a distance runner, while you don't have to sacrifice your speed to engage in endurance training.