The most important part of marathon training is the weekend long run. In most beginner programs you make a steady progression over the course of 18-25 weeks until you can finish 20 miles. Noobs may only do one before the race, veterans may have been doing them for years. The more long runs under your belt, the softer the dreaded marathon "wall" becomes.
But, the most common question non-runners have is "Have you run the full 26.2 miles in training?". They are shocked and surprised when the answer is "No!".
Almost universally you will find that marathon guides and coaches frown on running the full distance in training. The reason cuts to the simple essence of training:
Stress, then Rest, then Repeat
That's it. The stress event (long run, fast run, hill run, etc) tells your body to kick improvements into gear that will make it easier the next time. The rest allows the repairs and changes to take place before the next stress. Not enough rest, and you'll just break yourself down..i.e. "overtraining". If your a youngster, "rest" might mean run the next day, but go extra slow ("recovery pace"). If you're older, rest might mean "do nothing" or perhaps crosstrain (e.g. swim or bike).
For long runs, here's a partial list of the improvements that kick in:
- You grow more mitochondria to store glycogen and have more easy energy on tap.
- Your muscles learn how to metabolise fat (triglycerides stored in your muscles) when they run out of glycogen (carbs). This happens past about 14-16 miles. As this mechanism kicks in, the distance moves down to about 10 miles. You can tell when this is happening as a by-product of this reaction is something called 'ketosis'..and your post-run sweat has an unusual smell that is slightly chemical (ketones).
- Your joints and tendons harden up and adapt to the stress of the pounding.
- Your brain convinces your body to convert fast twitch muscles over to slow twitch for the long run.
- Your brain learns how to control and fire all your muscles properly for a good stride, even as your blood sugar lowers. (Preventing the 'falling apart' of your stride as you get tired'). Another way of saying this is that your running "economy" improves..i.e. you can cover the same distance with less energy.
- Your mind learns that you can overcome your feeling of wanting to stop ...you get accustomed to the feelings in your muscles past 13-14 miles and realize they can be tamed.
To return to our original question, the purpose of training for a specific race is probably because you want to maximize your performance given the amount of training time you are able to put in, with the natural limits on what your particular body can stand.
There is an optimum stress to generate the benefits above without requiring too long a recovery. A long recovery means you push out your next cycle, or you try to do another cycle when you are still tired and then injure yourself. Therefore, there is a trade off of stress intensity and recovery interval to get the maximum improvement for the amount of time put in with minimum chance of injury.
And so, since the above changes happen whether you run 20 or 26 miles, you don't need to run the full 26. Make sense? In fact, you don't run the 20 miles at your full race pace either. There are other changes to our body and mind we need to run fast, but we can do that with other types of runs and don't need to combine fast and long runs until the day of the race.
However, in the course of your training you also need to figure out what pace you should try for. How can you learn your pace if you never run the full distance at full speed? It is for this reason the "fast finish" long run was invented. Basically you run your long run at the normal training pace (which might be 1/min per mile slower than your idea of your race pace, and then, for the last 6-8 miles you kick it up to your goal race pace....and perhaps as hard as you can for the last 10 minutes.
By attempting the race pace after the first 12-14 miles of your run, you simulate the conditions late in the marathon and can see if you feel like the race pace will be sustainable for the entire race.
You also learn how to hold yourself back in the first half of the marathon...difficult to do when everybody else is passing you but critical to avoiding the "wall".
This Saturday I did one of these are part of my prep for the SF marathon. Here's a graph of pace and heart rate vs distance:
You can see that I kept my pace at 10:00 min/mile until mile 10 and then we opened it up, running at about a 9 min/mile
pace for the next 10 miles.