We tend to discount the easy run as "junk mileage" - is it just about having a big number at the end of the week? Or do those easy days actually contribute to our ability to run fast?
By my count >70% of my training days before Boston will be "easy". So, I hope it's important, otherwise I'm wasting a lot of time - but, how will these miles make me faster on April 20th?
Here are some specific physiological improvements that are made during easy runs.
Stroke volume: The heart is really just a pump sending oxygen & other nutrients carried by the blood to organs and systems that need those nutrients to function. Stroke volume is the amount of blood that is pumped with every beat of the heart. Higher stroke volume means that the heart doesn't need to beat as quickly - and studies have shown that stroke volume is improved due to time spent running/exercising rather than intensity of training (Daniels, 2005). So, even if you were to train harder - you wouldn't necessarily increase your stroke volume.
Muscle development: Easy runs increase the "number, size and distribution of the mitochondria" (Daniels, 2005). Mitochondria are the "only part of your muscle fibers in which energy can be produced aerobically" (Pfitzinger & Douglas, 2001) - they are basically little energy plants that use oxygen to produce energy for the cells they inhabit. Since 99% of energy is produced aerobically in a marathon it's important to have a lot of these little buggers. Exercise also increases the rate at which oxygen can be processed. And third there is an increase in blood vessels in the muscle - basically improving the distribution of oxygen to all parts of the muscle. Finally muscles get better at conserving glycogen, using fat for energy, and dealing with lactic acid through easy runs (Daniels, 2005).
Running economy: Although there are many ways to increase running economy - some researchers believe that the most important factor for improved running economy (both biomechanics and cellular economy) is by the amount of accumulated miles rather than the types of workouts that you do (Pfitzinger & Douglas, 2001).
There are certainly other components that are important for marathon performance that require more intense training (lactate threshold, overall speed, VO2 max) - but, we'll get to them later. For now, all I want or need are some nice easy runs (except for when Fed Hill gets out of control of course).