Why do you often recommend running slow to get faster?

Because it is usually the case that athletes and non-athletes alike have very poorly developed aerobic systems, which can only be developed starting at a relatively low heart rate (such as the MAF heart rate) and pace. As the aerobic system builds, faster paces at the same HR follow.

Even for people that are only interested in power, a well-developed aerobic system has many advantages. Anaerobic exercise such as sprints and powerlifting creates massive amounts of a compound called lactate. Lactate must be transformed into CO₂ to be expelled from the body. This process of expulsion occurs thanks to the aerobic system.

The more powerful your aerobic system, the more easily you will be able to recover from that particular set of lifts, for example, as well as from your workout session. In addition, a well-developed aerobic system has many other health benefits, such as reducing your stress levels, balancing your body’s pH, and generally making you more resilient to illness.

The aerobic system is actually so important to athletic performance that when athletes reduce their anaerobic training from 35% of their total training volume to just 20% their race performance actually goes up!

In Natural Born Heroes, for example, author Christopher McDougall writes about how the Dutch Olympic Speed Skater team owes their dominance in the 2014 Sochi Olympics to the preponderance of aerobic workouts (and the lack of anaerobic workouts) in their training regimen.