In the beginning of the book our impression of Cash is one of a very hardworking individual, the one with the best chance of being successful in life. We first get glimpses of his heroism when he builds Addie's coffin without stopping. Although it may seem like he is being insensitive, I believe that he is actually just trying to distract himself from the grief by working very hard, and that kind of release is pretty admirable.
Later in the book, Cash is heroic in his unwavering lack of notice that he puts towards his injury. Cash seems to know that complaining about his leg would make things worse for the whole family, so he endures that pain, saying that it doesn't hurt much, and makes them continue on the journey.
Cash also develops a lot during the journey. During the beginning he is wholly focused on building the coffin, but by the end of the book he turns into one of the most reliable narrators in the book. Cash manages to survive the whole journey, never complaining, and even questions some of the decisions of the other family members. One example of this is how he wonders if sending Darl away was really the correct choice to make.
Cash also is one of the members of the family who actually comes away with something, the "graphaphone". I think this suggests that he is more heroic because he did come out with something through a large amount of struggle and hard work, and because he lost a large amount of ability in one of his legs.
Overall, Cash is one of the most heroic characters in the novel and deserve the recognition that he gets by the end of the book as the main narrator.