The more...more expression links two ideas, and states that if one of them increases, so will the other one.
- The more interest rates increase, the more people will struggle to pay off their house mortgage.
- The more we go on destroying rain forest, the more problems we will have with global warming.
In your example, The sentence is saying that as marathon runners continue running, they will get more energy. (Personally I would have thought they would get more tired, but what do I know!)
Note that there are different forms of the expression:
- Marathon runners get more energetic the more they run.
- Marathon runners get more and more energetic the more they run.
- The more they run, the more energetic marathon runners get.
From these examples there are a couple of things to note. First, sentence 2 has more and more. The repeated more emphasises the increase in energy, but otherwise sentences 1 and 2 mean the same.
Second, sometimes we have more, and sometimes the more. The is added in cases where the subject hasn't yet been mentioned. In sentences 1 and 2 we have "Marathon runners get more energetic". In that form more is just an adverb connected to energetic. But in sentence 3, we have "the more energetic marathon runners get". Here the subject of the clause (marathon runners) is moved to the end of the sentence, so we add "the" to make it clear that we have the link between the two parts of the sentence. If we omit "the", we have this:
- The more they run, more energetic marathon runners get.
And we would ask ourselves, "more energetic runners get what?" We would be comparing more energetic runners with less energetic runners. It changes the meaning.