Let's see each point of your list...
- We start with an average human
So basically you are asking if training and knowledge can convert the average human into the best at a physical skill.
I can ignore the rest of the bullet points and tell you here that the answer is: NO.
Because for any sport there is selection bias on the athletes: only the people that has an inclination or some form of proficiency will try the sport at a proffesional level, and only those who perform well will be selected for world wide competitions.
This selection will translate to populations with biological adventages being overrepresented. For intance the average height of males in the USA is 175.7 cm (5 ft 9 in) [2014 data] but the average height of NBA players is 201.3 cm (6 ft 7 in) [2004 data].
So, it is expected that the average human will underperform compared to the best biologically fit individuals. Edit: Note that is given that such individual had decent training.
- The Neural Network in question is trained perfectly to understand the human body and its goal is to run marathon as fast as possible (minimize marathon running time)
I'll take the AI knows as well it is possible to know the body of the person it is training. Each body is different, perhaps the AI has considered the available information on all known athletes...
And I get you are using the work "understand" loosely here.
The AI you want probably isn't a simple neural network. You have stated that it has a goal, so, if you are using a neural network for this, the "smart" part is not only in the network itself but on the algorithm you are using to trai it.
- At the end of this training session, this human is so good in running marathon that he can win
Sure, he can win. But it will depend on who is this human running against...
Consider that there will be competitors that are have a better biologically fit body, and they have gone in exhaustive training too. Even if such training is inferior to the one provided by the AI, it doesn't need to be perfect training... just good enough.
Of course that is asumming all competitors had a similar preparation time and they are using equivlanet equipment. Edit: by "equivalent equipment" I mean, during the competition. You may disregard this - because it is standard - but clothing and footwear may mean a huge difference.
- This human does exactly as told by the Neural Network
- This human obeys everything the neural network tells him to do for a maximum of 5 years in row
5 years may not be enough. The training for the best performers in some sports may take 10 or even 20 years.
In other words, given a usual human we can determine what that human should eat, drink and how the subject should generally behave in order to run marathon perfectly.
The execution of the actions may be limited by the resources of this average human, such as access to equipment for intance. I wonder if this AI gives financial advice so that this human can buy the best equipment and best food.
And to think it knows what is the best equipment and food, would require perfect knowledge!
The AI needs to be trained for all possible food items, that would have to include all possible recipes, even that one your neighboor just invented yesteday and that new brevage that is coming to the market next month.
The AI needs to be trained for all possible training equipments and exercises. Again, this requires to know all that is in the market. Also consider that the best exercise may be one that requires apparatus yet to be invented.
The best we can realistically aspire to is an AI that is constantly learning.
Does this AI care for what is legal? It may tell this average human to steal the equipment, to use illegal drugs, and to undergo surgical body enhancements. And such action may not only break the law but also disqualify him from the competition.
And of course, killing all potential competitors with a better fit body would also "help".