People treat the marathon as a long half marathon. It is not. It is a different event with different rules, and the thing that catches most runners out is that the half lets you get away with mistakes the marathon will not.
BLUF: In a half you can pace on feel and survive a fuelling error. In a marathon, both of those will find you out in the last ten kilometres, and the cost of slowing down is far greater than anything you gained by going early. Decide which of two races you are running, then run that one properly.
Why the half forgives you
A half marathon is a long way. That is not in question. But it forgives you in two specific ways, and it is worth being clear about what they are.
The first is pacing. You can go on feel, and by the time it genuinely hurts you are usually close enough to the finish to hang on. There is a window near the end where willpower still counts for something, because there simply is not enough race left for the damage to compound.
The second is fuelling. Generalising, a half takes somewhere between an hour and two and a half. If you have paid attention to your nutrition in the days beforehand, your body is carrying enough stored energy to cover that. You can make a mess of your fuelling on the day and still come home more or less intact.
Neither of those is an argument for being sloppy. They are simply the reason a half marathon tolerates it.
Why the marathon does not
Go on feel early in a marathon and you will very likely pay for it in the second half. That much most runners have heard. What gets underestimated is how uneven the exchange is.
The time you gain by running the first half a little quick is small. The time you lose when it comes apart is not. A pace that felt comfortable at 10 miles becomes a grind at 18 and a walk at 22, and those lost minutes arrive in far greater number than the ones you banked. It is not a fair trade and it never has been. You are borrowing at a terrible rate of interest.
Add to that the duration. You are now out there long enough that stored energy alone will not cover it, so fuelling stops being a nice-to-have and becomes part of the race plan. The mistakes that a half absorbed are the mistakes a marathon amplifies.
So pick your race
In my opinion there are two honest ways to approach a marathon. Both are entirely legitimate. The trouble starts when someone wants the second one's result off the back of the first one's preparation.
One: you want to complete the distance
This is for anyone whose goal is to get round, or who has not covered the distance before. It is a proper goal and there is nothing lesser about it.
- Treat it as a long easy run. That is genuinely what it is.
- Conversational pace the whole way. Not "conversational for the first hour". The whole way.
- Expect the difficulty to come from the duration, not from the intensity. Being on your feet that long is the challenge, and it is enough of one.
- Fuel every 20 to 30 minutes, exactly as you have been doing in training. Race day is not the day to find out how your stomach handles something new.
- Soak up the atmosphere and enjoy it. You will not get another first marathon.
Run it this way and the odds of finishing, and of finishing in a state where you would consider doing another, go up enormously.
Two: you want the best result you are capable of
Different job entirely, and it starts long before race day.
- Train for it over a significant period. Not a few big weeks bolted onto whatever you were already doing.
- Train for it as though it matters. Treat it as an A race, with intent, and let the rest of the calendar arrange itself around that.
- Actually practise marathon pace. When the session says marathon pace, pay attention to the pace you are running. Not roughly. Not "that felt about right". Know what that effort is, so that on the day it is familiar rather than theoretical.
- Know your numbers, have a plan, and stick to it. The plan is the point. It is what stops you making a decision at mile six that you cannot undo at mile twenty.
- If there is more in the tank late on, use it. That is the right time to find out. Not early, and not to "see".
- Or, if you are going to have a go and see, accept what that means. It might come apart completely. That is fine, so long as you went in with your eyes open and you are not going to get grumpy about it afterwards.
The mistake in the middle
Almost every bad marathon I see comes from the gap between those two lists. Someone trains like the first and races like the second, then wonders why the wheels came off at 30 kilometres.
If you have not done the marathon-pace work, you do not know what marathon pace feels like, and "feel" on the start line is not the same thing as feel at three hours with a crowd carrying you along. Adrenaline lies early on. That is precisely when your plan is supposed to be doing the thinking for you.
Deciding which race you are running is not a lack of ambition. It is the single most useful thing you can do in the last fortnight before the start, because it determines every choice you make in the first ten kilometres, and the first ten kilometres determine the last ten.
The bottom line
The half lets you improvise. The marathon audits you. Both are worth doing, and neither is better than the other, but they ask for different things and pretending otherwise is what turns a good day into a long walk.
Pick the race you are actually running. Then run that one, all the way to the line.
If you are working out what your splits should be, our race pace planner will turn a target finish time into the numbers you need to hold. And if fuelling is the part you are least sure about, start here.