A reputation can grow into a legend, even in the person's lifetime, and the legend can become a myth. For a living legend, see the Sean Connery film, Robin And Marion. (Connery and Robert Shaw play, respectively, Robin and the Sheriff in this film and also James Bond and Donovan Grant in From Russia, With Love. Legends indeed.) When I told someone that there is still a Sheriff of Nottingham and that he sometimes breaks strikes, my friend replied, "Legends live!" For a legend becoming a myth, see Elliot S. Maggin's Superman.
Poul And Karen reconstruct a legend in The King Of Ys.
Quiz questions: How often are van Rijn and Adzel referenced in the Flandry series and where?
Literary question: Is Flandry's Legacy an appropriate title for the concluding volume of Baen Books' Technic Civilization Saga?
Flandry's legacy is indeed that some planets are better prepared to survive during the Long Night. He rescued Vixen from alien conquest. Vixen founded New Vixen and a New Vixenite contacted descendants of rebels exiled by Flandry. Although the Terran Empire fell, the Merseian Roidhunate did not move in to occupy post-Imperial space. Flandry had done a lot to thwart the Merseians and maybe contribute to their decline.
Cairncross hoped to be remembered through the lifetime of the universe (see here): a truly mad wish.