This refers of course, not to the manifesto of Dr. Timothy Leary, but rather Dr. Timothy Noakes or even more strongly in the writings of Dr. Jack Daniels (resident of Flagstaff, AZ not Lynchburg, TN).
Long Slow Distance (LSD) is the foundation upon which the house of fitness is built. The non-believers dare to call it "junk mileage" - like it's a waste of time at best, or at worst the refuge of scoundrels who care more of weekly mileage totals than running fast.
The folks who actually have done research argue that easy mileage assists with the following -
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).
Daniels defines easy distance as 65-79% of maximum heart rate. Many runners fall into the trap of running too fast on easy runs and therefore end up fatigued for more challenging workouts or races. They run with a group that's too fast for them or a group that's too competitive and races every easy run. That's why I would suggest that you should do most easy runs on your own, unless you have a running friend that you know won't try to push the pace.
In the 24 weeks I've planned for Chicago the first six weeks are entirely composed of easy runs. Even after week six over 70% of my mileage will be "easy". So, if you're coming back from a lay off or are new to running - don't feel like you have to hurry back to the track or racing. You can do a lot just by building your base of easy mileage.
Monday: 7 miles
Tuesday: 10 miles
Wednesday: 8.5 miles
Thursday: 0 miles
Friday: 4 miles
Saturday: 10.5 miles
Sunday: 13 miles
Total: 53 miles
Monday: 0 miles
Tuesday: 9 miles
Wednesday: 10 miles
Thursday: 10.5 miles
Friday: 8 miles
Saturday: 10.5 miles
Sunday: 17 miles
Total: 65 miles