Marathon pacing

If you've read my "Train fast, get slower" article, you may have gleaned the following bits of information:

The anaerobic system burns glycogen to fuel the muscles

The aerobic system burns the by-products of the anaerobic system (pyruvate) alongside fat. There is a limit to how fast it can burn the pyruvate. The leftover pyruvate breaks down as lactate which needs to be shuttled away from the muscle.

The faster we run, the more we use the anaerobic system and the more we rely on glycogen as our fuel source.

Combine the above with the fact that glycogen stores are limited to the extent that we barely have enough to last 26.2 miles, and we have the following implications for how we run a marathon:

We must have as much glycogen on board as possible before we start the marathon

We have to be as frugal as possible with our use of glycogen

We can address the first point by carbo-loading properly in the days beforehand. There are techniques for carbo-loading which I may discuss later, but there's plenty of information out there for those who want it. Some talk of hitting 'the wall', as if it's inevitable regardless of how well you carbo-load. This brings us to the second point.

The slower you run, the greater proportion of fat you burn, and the less glycogen you burn. Provided you have trained your fat burning ability with lots of long runs and providing you pace properly then you can make it to the finish line feeling good. Have you ever seen Paula Radcliffe hit the wall? Hmm maybe a bad example there as of course she hit an ancient Greek wall full on in Athens, when a stomach problem caused by anti-inflammatories prevented her carbo-loading properly. For all of her other marathons she's paced perfectly right to the end - not a brick in sight!

My personal experience has been varied. I've jogged a couple of marathons, Beachy Head and Salisbury, where I've been full of beans (pasta actually) right to the end. I've run a flat out marathon where my pacing was perfect and I ran the last 8 miles with no hint of a slow down (Abingdon). I've also scraped through a marathon where I pretty much ran out of fuel in the last 3 miles, but managed not to hit the wall (for reasons I'll explain shortly) - London last year. I've never actually hit the wall full on. I've just encountered a few bricks now and then.

The essential thing is to know your marathon effort and stick to it regardless. The thing to avoid at all costs is going off too fast and letting the lactate start to accumulate in your legs. Lactate for marathon runners is a complete no-no. In shorter races you can get away with a bit of exuberance and recover your pace as the lactate levels subside. In marathons, any excess lactate is effectively lost glycogen. You'll pay dearly at the end as you run out of your precious fuel source. It's all too easy when you're rested, tapered and fresh to get swept along in the crowd, so let me repeat:

Do not under any circumstances start the marathon too quickly!

Once you go over your marathon effort you are wasting fuel and bringing forward the inevitable collision with that wall. This might be at 20 miles, the traditional home of the wall, but if you run too fast you can hit it much earlier. I remember Jo Pavey hitting the wall in the Great North Run - a half marathon!

Notice I said you need to know your marathon effort. Knowing your pace in the cold of winter is one thing but if it's hot on marathon day you have to be prepared to slow down. In the heat, blood is diverted to the skin for cooling, depriving the muscles of precious oxygen and interfering with their ability to process pyruvate resulting in raised lactate levels. Fortunately for those of us using heart rate monitors for pacing, HR remains a pretty good guide for pacing in the heat: stick to the same HR as usual and you are naturally forced to slow down to a more appropriate pace.

I will deal with how to determine your ideal pace/effort in a later article, but to close I'll say a bit more about the wall and how to avoid it.

Your ability to replace lost glycogen is rather limited - for example it can take a couple of days to fully restock after a marathon - it's something that doesn't happen quickly. You'll see lots of advice about having the right fuelling strategy on race day with energy drinks and gels, but as necessary as this is, you're not replacing glycogen to any real extent. The main reason for consuming sugars on the run is to maintain blood sugar levels.

The muscles use glycogen: this is a chain of glucose molecules which can be stored in either the muscles themselves or, to a larger extent, the liver. The brain however uses simple glucose as its fuel source - this can be derived from glycogen or can be directly from the sugars you ingest. The brain, as the most important organ in the body, gets priority when it comes to the food rations. If the brain's sugar supply starts to dwindle due to dropping glycogen levels then it simply switches off non-essential systems to stop wasting energy. Unfortunately for us marathon runners, these non-essential systems include our legs: we hit the dreaded wall! By consuming sugars in the form of energy drinks, gels, power bars, jelly babies or whatever, we spare the glycogen and keep the brain's energy supply going. We might run out of glycogen and slow down at the end of the marathon, but we can keep burning fat for fuel and can keep going at a reasonable, albeit slower, pace.

I think that's enough for now. In future posts I'll try to explain the techniques I use for determining marathon pace (or effort), and I'll go into more detail about race day strategy. I've given you the why, you'll have to wait for the how.