"Hiking through these mountain ranges is very challenging." The active verb is "is". There is also a gerund, "hiking" which serves as the subject of the sentence. The subject clause is "Hiking through these mountain ranges", so even though "hiking" is a verb form, a gerund, it is not the verb in this sentence; it is part of the subject clause.
The rest of the passage contains other types of verb clauses, so you need to study those until you understand them! (:
