The marathon is a unique event: it’s long enough that not only does carb loading become an important key to your success on race day, but there are several other factors that are perhaps even more important.
These relate to your running efficiency and hence your ability to conserve this valuable fuel to make it last the distance.
The human body can only store enough carbohydrate in the muscles, blood, and liver (in the form of glycogen) for roughly 20 miles or 32 kms of running, especially at more strenuous intensities.
Run out of fuel and the inevitable crash or “hitting the wall” will happen, typically around the 32-35 km mark of the race. And you don't need to speak with many marathoners to learn just how common this is.
This is why as coaches we love those sayings like; " the marathon starts at 32km", and "the half way point of the marathon is 32kms".
The goal of the first 32kms is to arrive there in good shape, and enough fuel to get us through the final 10km to a strong finish.
Apart from ensuring you have a full complement of glycogen at the start of the race ( carb loading), we also need to ensure that we are using it efficiently.
Firstly, race planning is critical so we don't start too fast and use up too much glycogen during the excitement of the first 10kms. But even more important is in our preparation and training.
We do this by training our body to become more efficient by training it to utilise less glycogen as we run. Using less glycogen occurs at lower heart rates where our body will use O2 (oxygen) & fat as it's primary fuel source.
This system of energy production is known as your aerobic energy system and when well developed appropriately, can propel you at faster and faster paces, using the same amount of effort or fuel.
And this is where HR training is so critical to your marathon success.
Here's the science...
1. - the higher our intensity of effort the more glycogen we require, and the less fat is used (&O2).
2. - Also, the longer the duration of high intensity effort, the less glycogen we have available to us as it is depleted.
What these graphs tell us it two things;
1. In order to have enough glycogen to fuel us to the end of our marathon, we must rely on our aerobic energy system predominantly to maintain our intensity and conserve our glycogen to ensure it lasts the distance.
2. By training your body to produce more energy aerobically, thus more efficiently we can conserve valuable glycogen thus making it last longer. The less muscle glycogen you use, the more efficient you are!
Fat is the primary fuel source of the aerobic energy system. As we develop this energy system our body learns to break down & utilise fat more efficiently. It does this by improving oxygen transportation to the muscles, increasing vascularisation ( more blood vessels= more O2), and reducing lactic acid production ( removes H+ ions quicker that want to slow us down), and importantly, developing our mitochondria within the muscle that are responsible for the energy production from fat & carbohydrate oxidation.
So, how do we do this...??
This is where monitoring your heart rate in training is so important.
Go slower to get faster!
Ideally we want to train at the peak of our aerobic threshold to maximise the benefits of aerobic training & development. Whilst this heart rate threshold can be precisely measured in a VO2 Max test, where HR & blood lactate are regularly measured during efforts, it is easier & still very effective to use a simple formula as set out by the HR training guru, Phil Maffetone in the 180 rule.
As you become more familiar with this type of training, you will start to get a feel for precisely where this threshold is and when you cross it and see your HR spike up abruptly. Then of course you need to slow down!
Over the weeks and months of this type of training, you will know your aerobic energy system is developing as your training pace increases at the same Heart Rate. This is when you know you can run faster whilst still conserving valuable glycogen.
The good news is also that when you develop this system, all of your other shorter race distances will get faster too!
So at this early stage of your marathon preparation make sure you spend more time (70-80% at least) of your training time, developing this energy system. Thos that have the discipline to slow down and train aerobically for a period of time, will see great dividends down the road.