Training for marathon and 10000m is completely different.
For 10000m athletes don't have any problem about the quantity of fuel. The limiting factor is the power of the engine. For running fast 10 km, we need to train our body to produce high quantity of lactate, to tolerate this quantity in the fibres, and to remove it fast.
For marathon, instead, the main problem is about the quantity of specific fuel you can use.
We have, basically, two different tanks : one, very big, containing fat (the diesel of a car), another, smaller, containing glycogen (the super). Of course, faster we run, more super we have to use.
When an athlete comes from a career spent in short distances, he has an engine used to work with glycogen only. Everything specific is done with "super", the aerobic base is done with "diesel".
But, when we go to the marathon, the athlete must be able to create a specific mixture, putting together glycogen and fats, in order to last for the duration of the race. Top runners, able to run at the speed of their Threshold, use in one hour at that speed (about their PB in HM) all the glycogen they have, so their tank becomes empty.
The problem is that not all the carbohydrates can give the same quantity of energy, and that this energy is not the same : we have some of them able to burn very quickly, giving a lot of energy (it's the fuel we use in short distances), but also some able to burn slowy, producing a medium level of energy, but able to last longer time.
At the same time, not all the fats can burn slowly, only. We have some particular fat able tpo produce more energy than some kind of carbohydrates.
So, what the athletes has to learn, is how to create this mixture.
When there is not yet adaptation for long and fast run, at the supposed Marathon Pace the athletes use too much glycogen, so they finish their "super" far from the finish (may be about 30-32 km). In this case, the solution is running at even pace a little bit slower from the start, so the used fuel has a lower percentage of glycogen, and it's possible to last till the end.
But, if you want to be strong, you need, in a time of 5-8 months, to "teach" your fibres to run at the ideal Marathon Pace using, step by step, less glycogen and more fats, for the same speed.
We can control the change in the bioenergetic ability of the runner with tests of lactate, made on track at even pace for every test : for example, running 6 times 2000m with very short recovery, at speed every time faster, and controlling the level of lactate.
An example :
6 x 2000m in 6'20" / 6'15" / 6'10" / 6'05" / 6'00" / 5'55" includes a pace from 3'10" (2:13:38) and 2'55" (2:03:06), so for a top runner there is the control of what he can do during the race.
We need to search a Steady State for similar speeds, and when we find this is the correct pace for the athlete.
Example (lactate for the above times) :
1.7 mml - 2.0 - 2.1 - 3.2 - 4.8 - 7.6.
In this case, we have almost the same lactate level running in 6'15" and in 6'10". This athlete can run his full marathon in a pace between 3'07"5 (under 2:12) and 3'05" (2:10:05), supposing an even pace.
Other thing : we have athletes coming from short distances, and athletes starting with long distances.
The first typology is for "Fat runners", the second for "Resistant runners".
Also if the final goal is the same, the methodology must be very different :
In the first case, we need to EXTEND the duration, starting from the intensity they already have. That's means, for example, going for long slow run for adapting the structure and the mind to the marathon, BUT NOT FOR A METABOLIC REASON. In this case, the "specific training" of the marathon runner is to run fast (almost at marathon pace, may be 98% of it), once every 3 weeks, extending the distance (for example, 28 km - 30 - 32 - 34), while at the same time we use (always every 3 weeks) a long run of DURATION (may be longer than 2:30) at 80% of the Marathon pace.
The real training starts when the depletion of glycogen in the tank of the athlete is almost complete : if he's able to run 30 km at 3'12" pace (1 hr 36'), and in the last 4 km (for a training of 34) he slows down to 3'25" / 3'35" / 3'45" / 4', THE REAL TRAINING (UNDER PHYSIOLOGICAL POINT OF VIEW) IS IN THE LAST 4 KM, WHEN THE ATHLETE HAS TO FIND IN HIS BODY SOURCES OF ENERGY NOT KNOWED BEFORE.
In the case of a resistant runner, we need, instead, to qualify the duration, using progressive run, and also intervals on track more short, for mechanical reasons.
So, it's clear that, for an athlete coming from 12'56" in 5000m, we need to use this type of ability inside long fast intervals (for example, 4 x 5000m in 14'20" with 1000m recovery in 3'20"), but not with short intervals, because in this case we work in the opposite way of our physiological goal (that is to teach to use more fat at Marathon pace).
For an athlete coming from 45 km of long slow run twice per week, we need, instead, start on track putting a high volume of repetitions from 400m and 1000m, before going to longer intervals (that in any case are an important part of the specific period for everybody, if they want to reach their top).
About athletes running in 3 hours, of course this is not the way for training. These athletes, if sollecitated in their fastest speed, many times are not able to reac a level of lactate of 4 mml. This means they are able to use fats only.
That's the reason because to train 100 km is the most simple thing, if you are able to prepare the structure and the mind : there is no any mixture in the source of energy, since the athletes use fatty acids only.
Last important thing :
There is not a GENERAL SHAPE, but a SPECIFIC SHAPE. So, when you are in top shape for 10000m, you cant be in top shape for your marathon, and viceversa.
The preparation for one distance can be used as "special" preparation for the other, but after we need to move in the specific direction.
This is, for example, what I did last year with Wilson Kiprop : he ran Prague Marathon (9th May) in 2:09:09, only for controlling his aerobic base, looking at 10000m. At the end of June he won Kenyan Champs with the new WR in altitude (27'26"), one month later African Champs beating Kipsiro in the last lap (14'03" + 13'29"), and in this period he maintained long run of 30 km in 1:36:00 about, once per week.
After this, assembling long endurance and specific speed for 10000m, we moved to HM, and he won the title in Nanning, using all the different types of training carried on before.