But it is also possible to use '지루하다' as a verb here, right? Or is it used in a different context?
You are right. "이 긴 책은 지루합니다." is now accepted.
아니요, 이 긴 학교는 재미없습니다
Dziady cz. 3
Why is it not 길은?
Because of the rule that ㄹ is removed and replaced with ㄴ, since the infinitive form of "to be long" is 길다.