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Why is the 20-miler so popular?
Given the overwhelming scientific evidence against long runs of over 3 hours, why are they so prevalent in marathon training?
First, many people have a mental hurdle when it comes to the 20-mile distance. The marathon is the only race that you can’t easily run in training before your goal race. Therefore, much like the 4-minute mile and the 100-mile training week, the 20-mile long run becomes a mental barrier that feels like an obtainable focus point. Once you can get that 2 in front of your total for the day, you should have no problem running the last 10K, or so your mind believes. Unfortunately, this just isn’t true from a physiological standpoint.
Second, the foundation for marathon training still comes from the 1970′s and 1980′s at the beginning of the running boom. Marathoning hadn’t quite hit the participation numbers it has today (you could sign up for most marathons, including Boston, the day before the race) and the average finishing time at most races was closer to 3 hours (today that number is over 4 hours). As such, the basis for how to train for a marathon came from runners who averaged close to 6 minutes per mile for the entire race. Therefore, 20 and 22 milers were common for these athletes, as a run of this distance would only take them about two-and-a-half hours to finish at an easy pace.
Moreover, the 20-mile distance is synonymous with “hitting the wall” or “bonking” during the race itself. “Hitting the wall” frequently occurs at 20 miles because your body can store, on average, two hours worth of glycogen (fuel) when running at marathon pace. Two hours for a 6-minute per mile marathoner occurs at almost exactly 20 miles.
In short, the basis for a lot of our understanding of marathon training is passed down from generation to generation without regard for the current paces of many of today’s marathoners. Therefore, we also need to reassess where the long run fits into the training cycle and how we can get the most benefit from training week in and week out.