OK, I've been in my own head too much lately, and I'm not sure what my answer is to this. I guess one answer (for the musician, at least) is to hopefully leave behind some material of interest, either music performed, documented and/or composed. I also think one's legacy is represented by how we deal with our everyday interactions (you know, that karma stuff).
Responses don't have to be to be music-related. I think we all would like to leave something of significance behind when we depart. I'm not sure what this will be for me; as a musician I have much more work ahead. For some folks, their legacy might be primarily about raising/supporting a family.
As far as the big picture, IMHO the answer lies not in the battle itself, but also how you choose to fight the battle.
This thread is wide open. What are your short- and long-term goals/hopes?