Legends have been defined as historically grounded fairy tales, folk tales and historical narratives with embellishments. A person is a legend if they stand out or if they perform incredible tasks. Robert Neville is a legend because he does stand out. In the film version, he becomes legendary after his death for his contribution to the cure. But I think in the context of the book, he is a legend because he represents the last of the human race prior to the spread of the vampirism virus.
Legends are often based upon actual events. Over time, they are embellished. Sometimes, they are embellished to the point that they become verisimilar (only resembling truth). If they are further embellished, they become mythological. In this case, the tale may be based on historical events, but it has become so impossible to believe that it becomes a myth.
I don’t think Neville’s case has become myth. It is still verisimilar even as a fiction. It is possible that a virus could spread resulting in this apocalyptic scenario. It is unlikely but possible. So, I would say that Neville is a legend because he does stand out in his own time. Until he meets Ruth, he is alone in a world of zombies. He becomes “legend” when he dies because he is the last of his kind. He is the last of the old world of humanity. That old world has now become legend.