Montag is moved more by his emotions than by any deeper desire to be a "hero" as commonly defined; he doesn't take direct action until he is forced into a corner, and he doesn't really try to alter the society for the better. However, this is a function of his character arc; he is waking up, discovering his individuality, and learning how to think instead of what to think. By the end, it is clear that he doesn't view himself as any sort of hero, just an unlucky man with little to offer:
[Granger said] "Walk carefully. Guard your health. If anything should happen to Harris, you are the Book of Ecclesiastes. See how important you've become in the last minute!"
"But I've forgotten!" [Montag said.]
"No, nothing's ever lost. We have ways to shake down your clinkers for you."
"But I've tried to remember!"
"Don't try. It'll come when we need it."
(Bradbury, Fahrenheit 451, Google Books)
Although Montag thinks he has failed -- he lost his friend, his wife, his life, and his job -- in fact he has succeeded in becoming an individual with something of value to offer future generations. His actions may have served to wake up some of other other citizens, and those who escaped or survived the destruction of the city will remember his defiance of societal pressure and rethink their own lives.