I remember reading a quote from one of running's iconic coaches, Lydiard, or someone of his stature, years ago. Paraphrasing, he said the long run does not start until seventeen miles. I thought it was a ridiculously elitist thing to say. But I came to understand why he said it. Runs of increasing length run moderately fast are about increasing connective tissue and muscle strength, greater mitochondrial density, capitalization, and increases in myoglobin, heart muscle strength, and running efficiency. But around seventeen or eighteen miles, if running fast enough, the runner will have consumed a substantial portion of her glycogen, the carbohydrate stored in muscle for immediate use while exercising. The body uses a mix of fat and glycogen for energy production. At rest, while walking or running slowly, the primary energy source is fat. As effort and speed increase, the body derives more energy from glycogen, which is more efficiently converted to power the muscles. An ultra-marathoner can run past 26.2 miles with no energy issues because she is running more slowly and getting more energy from her unlimited fat reserves. However, a marathon's point is to run for a time goal. At that pace, glycogen depletion becomes an issue, and the runner can hit the infamous Wall. The long run, which is longer than 17 miles, is crucial for the marathoner. During this training, the marathoner adapts her physiology to the demands of running 26.2 miles and her biochemical energy pathways to run faster while burning more fat proportionally.
I understand and respect what that number represents as someone who has run past 17 miles many times. That's why I cannot bring myself to call my seven miles a long run. "Long run" holds a special place in my mind as training, sometimes testing, oneself for the Wall. It is a training modality reserved for the marathoner.