Common Limiting Factors in the Marathon
... and how to conquer them to run your best marathon
Endurance athletes the world over are familiar with this concept, having experienced it on at least one occasion--the narrowing field of vision, exponentially increased effort and deep fatigue, such that you feel like lying down in the ditch and taking a nap. It's the layman's term for hypoglycemia, or a low blood sugar. Like cramps, it will dash any hope you had of doing your best, effectively ending the race for you. But unlike cramps, it's much easier to predict and prevent, mostly because it's relatively easy for us to study exercise metabolism in the lab. Although the field of carbohydrate metabolism continues to advance, we've unlocked most of the important parts that apply to everyday runners.
The first thing you must do to prevent hypoglycemia is to be sure you start the race with at least normal glycogen levels in your muscles and liver, which is the organ responsible for maintaining the blood glucose concentration. There's much debate now about whether traditional "carbo-loading" is necessary; without going into that whole argument, the minimal requirement is that you toe the line with at least normal concentrations of glycogen in your muscles and liver. Increased levels may or may not add value, but less than normal is detrimental.
Part of your training program should be making sure you ingest sufficient calories to meet the increased expenditure from all the training you're doing, and that means not only eating regular meals, but depending on how much training you're doing, eating supplemental food (not necessarily "supplements," but any food) to ensure you meet your energy demands. If you're doing that, you're in all likelihood starting off with normal levels, especially since a normal diet for most runners tends to be high in carbohydrates.
So starting off normal is what you must do before the race, but once the gun goes off you then need to ingest anywhere between 30–60 grams of carbohydrates per hour to prevent your liver glycogen levels from getting too low. The problem is that, as we carry on exercising for many hours, we tend to rely less on the carbohydrates stored in the muscle and more on the carbohydrates from the liver. We do also tend to use more fat, but even as we increase our fat usage we still use more blood glucose over time, and that's why the liver needs a little help in the form of what we can ingest to make sure it doesn't get too low. Once the brain senses that it might be getting too low, it has its ways of making you slow, and if you choose to ignore those signals then you'll most likely bonk.
I realize we don't think in terms of grams of carbohydrates, so let me put that number of "30–60 grams per hour" in context. A typical gel product has between 20–30 grams of carbohydrates, while sports drinks contain about 10–12 grams per cup that you might receive on the course (around 200 mL or 6–7 oz.). Therefore, one gel and two cups of sports drink per hour will put you in the right range, and most runners' thirst drive will cause them to ingest anywhere between 400–800 mL (14–28 oz.) per hour depending on the conditions. Of course you're not limited to gels and sports drinks, but they'll be the most readily available and easiest to consume.
Trying to go the distance in the absence of ingesting any carbohydrates is risky. Some can do it, but it will depend entirely on their genetic predisposition to burn fat during high-intensity exercise, as well as their training status. Chances are you're not one of those people, however, and taking in some carbohydrates along the way will therefore be beneficial, if not essential. It might be less as opposed to more, and that's where experience starts to pay dividends, because it might be a few marathons before you fully understand what's going to work best for you and in what amounts.
THE BIG PICTURE
Bear in mind that these are what I consider to be the most common issues runners face when trying to achieve their best in the marathon. Many of you might have alternative lists that relate directly to your experiences, and that speak to the real challenge of the distance itself. Over the shorter races, even up to the half marathon, there's much more predictability. In other words, there are far fewer things that will affect your outcome over those distances, with training and your talent predicting most of your success. For example, over the shorter distances issues like metabolism really become inconsequential because racing over those distances doesn't represent any real challenge for that part of your physiology. No one's going to bonk in a 5K or 10K race, and probably not even in a half marathon. But when we add in the full marathon distance, things get a bit gray. Problems due to minor issues like a lack of core stability that will never present over the shorter distances suddenly become amplified over the marathon.
Good, solid preparation and training is always the best remedy, but recall that's only the first half of the problem. You must execute an appropriate pacing strategy to run your best on the day. That means being realistic and owning up to the fact that, perhaps, due to a niggling injury you weren't able to complete as many long runs as you planned, and then adjusting your time goal accordingly so that you can run an even or negative split.
An important factor, however, is that marathon training takes a tremendous amount of time, energy, effort and commitment. So much so that we can realistically expect to aim for only two good races per year at that distance, and so I fully understand the situation we often find ourselves in: It's been a long and hard four months of dedicated training, and you picked up some minor injuries that made you adjust your training plan, or work got out of hand and you didn't get in as many miles, but the day is here and you might not be able to attempt another marathon for 12 more months. In the end you decide to just go for it because of the dynamics of your situation--your need to qualify for Boston, your long-standing goal to break 3 hours, your standing among your running buddies.
I would like to urge you to reconsider that and to take a more conservative approach. The marathon is as much a game of patience as it is endurance and strength, and not just on the road. Expect to change your goals along the way, and more importantly, be OK with that. You're a runner, after all, and I hope you won't be hanging up your shoes any time soon, which means there'll be another marathon in your future. And all the training you've done prior will only benefit you.
The final point I'd like to share with you here is that, even with our best-laid plans, even with the most accurate and scientific information available to us, getting everything to go exactly right on the day still requires a little (or a lot of) luck. For most of us, racing a marathon really does stretch us to our limit, and predicting what will happen during such a complex situation is terribly hard and often imprecise. So prepare well, pace accordingly, and most importantly, enjoy the scenery.
HOW DO I KNOW WHAT PACE TO SET FOR A MARATHON?
To set a realistic time goal we really need to understand our potential and what's possible for us with our current level of training. We tend to overestimate our abilities, and in so doing create a situation that will likely produce only failure. So how fast can you go? The best predictor of your next marathon, even if you've run many before, is your most recent time over any shorter distance. If your half marathon time two months before the marathon is 1:28, you're not going to break 3 hours. If you're very well-trained, you can expect to multiply your half marathon time by at least 2.1, but you'll need to add more than that if you're not as prepared and likely to slow down more during the race. Similarly, to break 3 hours you probably need to be able to run a 10K in less than 40 minutes. You'll find numerous race prediction charts online, including our Pace Tools here. Regardless of what you set as your goal when you started training, at some point you need to adjust that goal to your ability as revealed in tune-up races leading to the marathon.