Learn Russian: Трасса этого марафона очень длинная.

This is the genitive case, which is often used to translate the English word "of" or to show possession. Here, it means "of this marathon". Because марафон is a masculine noun, the demonstrative pronoun этот changes to этого and the noun adds an -а to become марафона.

Why does the adjective длинная end in -ая?

Длинная (long) describes the subject of the sentence, трасса (route). Since трасса is a feminine noun, the adjective must match its gender and case, taking the feminine nominative ending -ая.

Where is the verb "is" in this sentence?

As a quick refresher: in Russian, the present tense of the verb "to be" is almost always omitted. So instead of saying "The route is long", the sentence literally translates as "The route of this marathon very long".

How does трасса fit into the context of motion and routes, compared to a word like дорога?

While дорога is the general word for "road" or "way", трасса refers to a specific, designated route, track, or major highway. For a marathon, it specifically means the racecourse that the participants will move along.