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  • Writer's pictureClem Dixon

In the Long Run

Updated: May 18, 2021

First in an occasional series of articles about distance running.

How many decent marathon runners do you know who have never really done themselves justice in the half? Sounds like an odd question, right? Switch it round and ask how many people you know who don’t seem to have been able to translate their performances in the half to the marathon and it seems like a more normal question, and your answer is probably that you know loads of such people, perhaps you are one, in which case, read on!

Take a look at the following table. For a list of runners, some more famous than others, I have used their half marathon PB to predict the full marathon time that they should be capable of and have compared that with their actual marathon PB. The table is ordered by the percentage discrepancy between the two, so those at the top are achieving, or even surpassing, their potential; the further down you go the bigger the shortfall between potential and actual performance.

If you are not listed in the table but would like to know where you would slot in: to predict your marathon time multiply your half PB by 2.0849. Why 2.0849? A generally accepted formula for predicting a time T2 for a distance D2 from a performance of T1 for a distance of D1 is: T2 = T1 x (D2/D1)^1.06. So it is 2.0849 because that is 2 to the power of 1.06, but you didn’t need to know that.

The purpose of the list is not to “name and shame”, there are all sorts of reasons why someone might be near the bottom, perhaps they have never really seen the marathon as a priority, or perhaps their marathon times are still improving but lag behind their times for the half. The purpose of the list is to demonstrate that, outside of the elite, under-performance in the marathon is common.

Why is this? My contention here is that the main reason is that not enough emphasis is given to “the long run” in training.

We need to talk about energy sources, apologies to those who already know this: the energy that you use up during a marathon comes from two sources: glycogen and fat. Actually, there is a third potential source which is calories that you take in during the run but these only have a marginal effect, that subject might crop up in a future article. Glycogen has the advantage of being readily convertible to energy, it is there as soon as you need it. But there is only so much of it and during a marathon it will run out. Fat on the other hand is only available slowly. It is as if you are in an old-fashioned shop and the guy behind the counter is hard of hearing, when he finally works out that it is calories from fat that you are looking for, he ambles off into some backroom and rummages around for ages before finally giving you what you want, grumbling if you ask for too much. On the plus side, even if you are as skinny as a rake, the fat will not run out, not in a single marathon anyhow.

How far can you get on glycogen alone? All runners are different and it is more complex than the glycogen stopping all of a sudden, but an often quoted answer is about 18 miles. The important point is that it is way further than a half, and nothing like as far as a marathon. Marathon runners talk of “hitting the wall” (cyclist and triathletes of “bonking”), this can cover a range of things, mental as well as physical, but a key component is running out of glycogen and having to shout at the old geezer in the fat store.

But there is good news, you can train your body to get more efficient at burning fat (and you can increase its capacity to store glycogen). How do you do this? Like everything else you train your body to do it by doing it; you run in a glycogen depleted state. You keep going to the fat store and the shopkeeper starts to recognize you as soon as you walk in and knows what it is that you are after. This is what the long run is for.

Perhaps at this point you are thinking: what a lot of words to tell us what we already know. Everyone knows that a marathon is a long way, and that you have to run a long way to train for it, and that is what we are already doing! Are you though? Are you really? Many folks make one or more of the following three common errors with their long runs: they don’t do enough of them, they aren’t long enough, and they do them too slowly. Let’s look at each of these in turn.

For many the mindset of the long run in marathon training is to gradually increase the distance until it gets close to the magic 26.2 and then to tick the box satisfied that you have got the distance in you. This is not the point of the long run, the point of the long run is to train you to burn fat more efficiently and to do that you have to do it again and again and again, just like any other training. Of course if you are not used to running distances of 20 miles or more you shouldn’t try to start doing them regularly from scratch, you run too high a risk of injury, but the process of building up the distance should be got out of the way before the real work begins.

When people who are training for a marathon talk to me about their long run I like to ask what distance they are doing; it is amazing how often you get a straight-faced answer of something like 15 miles! They are not reaching the glycogen depletion stage, so it doesn’t count as a long run!

When you do get to the glycogen depletion stage you need to be running reasonably fast, if you are doing little more than jogging then you are not putting sufficient demand on your fat burning system. Like everything else it is a trade-off, the closer to target marathon pace and distance you get the more benefit you will get, but the longer it will take you to recover from the run. If it is more than a week until you can do it again then it probably wasn’t worth it.

So what sort of long run should you be doing? My personal favourite is 23 miles as follows: the first 3 miles at a jog, don’t even start the watch; the next 10 miles at 20% slower than target marathon pace; the last 10 miles at 10% slower than target marathon pace. The 10 miles at 20% should feel really easy, but don’t be tempted to go faster, in the race itself you will be tempted to start faster than you should so this is good mental training. The last 10 miles shouldn’t feel quite so easy! If they do then you are perhaps being too conservative at setting your target marathon pace. To give you an idea of what these paces mean for you here is a table:

The details of the run outlined above are not what is important, what is important is the long total distance and the loading of the back end of the run with the faster speed. A few years ago, I was overtaken by Matt Winn-Smith towards the end of a 20 mile road race. And not simply overtaken, it was like when you are doing 70 on the M20 and the Eurostar goes past you. At the time Matt was our club’s leading endurance athlete although he specialized more in the ironman than the marathon. Talking to him afterwards it turned out that he had used the race as a training run, taking advantage of the traffic free course, drinks stations and mile markers. He had jogged the first 5 miles and then increased his pace by a minute a mile after each set of 5 miles. Knowing him he had probably done 50 miles on the bike before the start.

To labour the point, imagine two runners, both have decent half marathon performances under their belts and both are having a crack at the full marathon for the first time. Our first runner starts his marathon training schedule 3 months before the race, it is based heavily on what he did to train for a half (reps at the track, tempo runs and the like) but he also starts to increase the length of his weekend long run starting form a 10 mile base. He gets to 3 weeks to go and has been regularly running 16 miles at close to target marathon pace. A nagging doubt that the last 10 miles might not be as easy as the first 16 persuades him to run 20 miles; he starts as if he was doing 16 but fades badly in the last 5. Disappointed, he has another go a week later; this time he starts off even faster and fades almost as badly but knocks 5 minutes off the overall time so pats himself on the back and tells himself that it will all be alright on the big day.

Our second runner gets to 3 months to go having already built up her mileage to the point where she can regularly run 20 miles. She scales back on her speed-work to give more emphasis to her long runs and between 3 months and 3 weeks to go she does 15 23 mile sessions of the type described above. If you accept that the most important aspect of training for both these runners is to get better at burning fat, and you accept that that doesn’t happen until after 18 miles, then you will see that our first runner has only really done 4 low quality miles of what really matters, whereas the second runner has done 60, and of higher quality.


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