An adjective is a word that is used to describe, qualify a word in a sentence.
Therefore, from the given sentence, there is no adjective as there are only nouns, verbs, prepositions, etc.
The sentence describes the routine of marathon runners before a race but does not use an adjective, rather making use of nouns such as "potatoes, pasta, carbohydrates, marathon", etc.
Examples of an adjective include:
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