Noach may have been a tzadik only relative to his peers, or in spite of his peers, but whichever nuance is correct, shouldn’t the pasuk have said he was a tzadik “b’doro” – why the plural, “dorosav”? Noam Elimelech writes that each generation has a particular mitzvah which it must perfect. Noach’s life spanned multiple generations, and he was able to master the particular mitzvah challenges unique to each of those doros.
I think there are two messages here. First, the obvious one: Don’t be the general constantly fighting the last war. Imagine a person who is a big tzadik and watches no TV, but then along comes the internet and he is glued to his browser 16 hours a day. That person was the tzadik hador, but not the tzadik b'dorosov – the world moved on to new challenges and he could not keep up. Second, the more subtle one: imagine a guy who works at perfecting his mitzvah of talmud torah, learning without interruption for 16 hours a day through yeshiva. Along comes marriage, job, child #1, child #2, etc. and the guy is still learning 16 hours a day while his wife gets no rest, his kids run wild, and he has no income or career. That person is working on the wrong mitzvah for his dor. He is putting all his kochos into talmud torah, while ignoring the opportunity and signs Hashem has given him that he should be working on perfecting the mitzvah of chessed (not to say he should ignore talmud torah k’neged kulum, but the experience must change). Again, such a person is the tzadik hador, but fails the test of being tzadik b’dorosov.