People often ask me if I hit the wall when I run my Marathons, it is one of those things that commentators love to talk about - they make it all sound so dramatic! But what does it actually mean to 'hit the wall'? Your body's main source of energy while running is glycogen, your body can only store enough for about 18 miles of running, hitting 'The Wall' is the term given when you your body runs out. This leaves your legs heavy, lethargic and you gasping for breath.

To understand the concept better you need to know a little bit about human physiology. Your body has two main sources of energy, glycogen, and fat. You are always drawing on both of these sources, and they both have their advantages and disadvantages.

Energy from glycogen is available quickly and cheaply in terms of oxygen requirements, but you only have a relatively small store of it. You have a much more plentiful supply of energy from fat, but it requires more oxygen to extract the same amount of energy.

During any form of exercise your body will extract energy from both of these sources, but it will adapt the ratios depending on the intensity of the exercise you are doing. If the exercise you are doing is lower intensity, a greater proportion of energy will come from fat. For higher intensities, a greater proportion will come from glycogen.

Running is one of the highest intensity forms of exercise you can get, so you are relying almost exclusively on glycogen for your energy supplies. When your body runs out of glycogen, your body is forced to extract all of it's energy needs from fat. The human body is not well adapted to doing this, it doesn't normally need to be.

This is where the long training runs come in, your body will completely run out of glycogen at around 16-20 miles. By running this distance in training you are conditioning your body to improve it's ability to extract energy from fat. You can't train enough to avoid hitting the wall, it is the boundary of human physiology. But you can condition your body so that you don't 'hit' the wall as hard by making your body more efficent at extracting energy from fat.

The term 'hit' suggests a very specific time when suddenly your body runs out of glycogen. The reality is that it is a gradual transition that starts in the region of 14 miles. You will notice that you are starting to feel more fatigued at the 14 mile mark, but you probably won't immediately associate it with the 'wall'. What I find that usually happens is that you will continue on unaware, the fatigue will get worse until suddenly you notice it and much more aware of it. This is the point referred to as the 'wall'.

This is ultimately what makes the marathon distance so hard, crossing that threshold. By that time, your legs are heavy and tired, your breathing suddenly becomes very heavy, your blood sugar levels are low, and you feel lethargic. So you have the possibility of feeling terrible, and you probably won't care a bit about finishing. You continue on to 20 miles, but you still have 6 miles left, which isn't a huge distance normally, but it will be the longest 6 miles of your life.

This is why long runs are so vitally important to marathons, by pushing against that boundary you are putting your body in a situation where it needs to rely on fat. Don't underestimate the effect glycogen-depletion will have on your body. It is hard to convey the effect it has using words alone, you need to appreciate it first-hand to properly appreciate it's effects.

How long should your long runs be? This is a very individual question. The minimum I would reccomend is 18 miles, even for first-timers that haven't done much running before. The problem with running for this distance and exhausting your glycogen supply is the increased recovery period. The recovery time needed for a run up to the distance of 15 miles rises fairly linearly with distance, but when you get to the region of 18 miles, the recovery time starts increasing exponentially.

There may only be four miles difference between 16 and 20 miles, but the difference in recovery time is huge. It's always a trade-off in this situation between maximising the distance of your long run, against the increased recovery time/increased susceptibility to injury/lowered immune system. So, pay attention to your long runs!