There is a method behind the madness. Six miles is a big difference, as you will find out while you increase your miles during your training program. I understand why runners want to run the full 26.2 prior to race day. They want to know how their body will react, what type of mental challenges will come during that last 10k, and they want to know they can do the distance before race day. How embarrassing would it be to drop out or have to walk right when the crowd begins to gather around you cheering and encouraging you to keep pushing, your almost there, and how amazing you are. Right? Yeah, I get it.
But, Nicole, you run twenty-five and thirty miles and then go and do it again the next weekend one little marathon won’t be a problem for me. Here’s the thing, I have spent years training my body to be able to run those distances and then recover from them quickly to do it again the next day or the next weekend. So in less you are planning to add three months to your training program, I suggest you don’t go over the twenty miles.
The primary reason that it is not a good idea to run the full marathon distance prior to your race is that it takes too long for you to recover from the run. For first time marathon runners it can take anywhere from 10-14 days to recover. That’s two weeks where your one long run is having a major impact on your training level and quality. One run is not worth that.
Running the full marathon distance during training will not give you the best idea of what your time will be during the actual marathon because you wouldn’t taper for the training run. By not tapering down your miles and intensity during the 10-14 days prior to running the marathon distance, you are running on a depleted body. In many cases, it will be more difficult than actually running the race.
You can get a firm understanding of the mental challenges you will be facing during a marathon when you run your 20 mile long run because you will be running that without a taper as well. Twenty miles in long enough to give you the feel of running the full marathon without the cost of actually doing it. It gives you the chance to implement the fueling plan you will be using on race day, to understand your hydration needs, and to get an idea of pushing past the point where your mind is telling you to stop.
If you can run twenty miles with a depleted body, running 26.2 miles with a fully recovered (from the taper) and trained body will not be a problem. It won’t make it easy. There will be challenges, but you will make it to the finish line.