For years, the venerable long run has been the main staple of the marathon training program. The physiological benefits are well documented, but there is the added psychological benefit which comes from asking your body to accomplish something it may never have done before. The idea of completing your first 20-miler brings with it much apprehension, but also much anticipation. And the satisfaction of completing the distance can get stored away and called upon when you are in the midst of doing the actual marathon and needing to draw motivation from your mental well.
Having said that, what I am about to discuss will seem like heresy, but it is based in well-researched sports science; Unless you can complete a 20-miler in three hours or less, you may actually do more harm than good to yourself. To put this in better context, most research has found that running for a period of 2-3 hours returns the most physiological benefit, whereas anything beyond the 3 hour mark can start to have deleterious effects; the science behind the long run is to stimulate adaptations; it is a means to an end. Imagine what it would be like if we always had to run for the distance we were training for. If that were the case, how could you ever possibly recover if each Sunday you had to run 26.2 miles? The point is, you have to consider time on your feet and the pounding your body takes as much as you have to consider the miles that you run.
In most beginner marathon schedules that you find online, many of them include long runs that eventually build up to at least a 20-miler; some even higher than that. Most sports science research, along with many nationally recognized distance coaches, suggest that the long run should make up anywhere from 25-30% of your overall weekly mileage. It should compliment your training regimen, provide the appropriate stimulus for adaption and not require more than a couple of days to recover from. If this news is causing you some anxiety in that you may never get to 20 miles except on the day of the marathon, please don’t dismay; this is why we have the other days of the week to train. Adaptations are not the result of any single run (long or otherwise), but, instead, are the cumulative effects of all the many runs you do. All the many runs over the weeks leading up to the marathon, along with the appropriate rest and recovery, will get you to the finish line.
If after all this, you still feel compelled to attempt the distance, then I can only caution you to be careful and listen to your body. We are all adults; my responsibility as a coach is to advise and provide you with enough information to make smart choices about how much your body can handle. While you might be successful in completing a 20-miler, your body may take issue with you at a later time by developing pain somewhere you didn’t expect or become run-down because of too much accumulated fatigue. Please, please be cautious!
All the best,