According to the old man,"what is a personal legend"?
One's personal legend could be that inner voice (soul?) that speaks. I've taught my students (and myself) that it's this voice that helps us to identify our purpose in life and carry us towards this purpose. In the story, the struggle between those that heed the calling of the voice (Santiago) and those that don't (crystal merchant) is a main thrust of Santiago's quest. How does one reconcile this notion of following one's heart with the sometimes equally strong pull of accepting one's lot? This idea is a theme which can be explored in depth because both ideas merit consideration.
The idea of a "personal legend" suggests that it is a quality that is innate in all. Some might argue that your personal legend is latent and that it can be coaxed out over time. Others might argue that it needs to be discovered first. Either way, it's one of the driving points of Santiago's quest.