According to data derived from statistics made available by Running USA, most runners training for the marathon are averaging anywhere from 8 to 11 minutes per mile on their long runs (training paces for a 3:30 to 5-hour marathon race finishing time). At a pace of 9 minutes per mile, a runner will take roughly 3 hours to finish a 20-mile run. While there is no doubt that a 20-mile run (or longer) can be a great confidence booster, from a training and physiological standpoint, they don’t make much sense. Here’s why:
Most coaches and exercise scientists now know that your body doesn’t see a significant increase in training benefits after running for 3 hours. The majority of physiological stimulus during long runs occurs between the 90 minute and 2:30 mark. This means that after running for 3 hours, aerobic benefits (capillary building, mitochondrial development, myoglobin levels) begin to actually stagnate or decline instead of improving. Therefore, doing a marathon as a long run builds about as much fitness as your normal 20-22 mile run.
While you might be OK with not building any additional fitness when doing your fun marathon long run, the downside is that the longer you run, the more you increase your risk of injury. While the extra 6 miles to go from 20 miles to the full marathon distance might not seem like much, muscle fatigue increases exponentially with every mile. During those last 4-6 miles, your form begins to break down, your major muscles become weaker, and overuse injuries begin to take their toll–even if you’re running easy. This risk of injury over these last few miles is more prevalent for newer runners whose aerobic capabilities exceed their musculoskeletal readiness. In simpler terms, a newer runner’s body isn’t ready to handle the kind of stress that their lungs can withstand.