Not sure what makes you draw such a conclusion, it looks pretty much out of place. Since there's no deity or "higher plan" or any of that as you say, then we can't really have any duty, same as a cockroach has no duty when it runs around the kitchen. There's no reason to think our running around should be any better than a cockroach's.
How can you possibly say such a thing?
There is none, if you don't want to give it any. A cockroach doesn't know what it is or what it does, or why it does what it does. We humans do. We have the extraordinary capacity to reflect upon our actions and their consequences, to look around us and ask questions about the world, about ourselves, our fellow human beings... we are able to feel love and hatred, compassion and ire, pride and despair, etc., etc.
Only if we accept reality for what it is will we truly excel as human beings. I find it an extremely limited, sad and depressing view that we are merely creatures made by a superior supernatural being, and that our ultimate duty is to please him and adore him so that he may grant us access into his happy place, and not send us into eternal torment.
Let us rise to the occasion and accept that we are evolved apes in a tiny spec of dust within this immense universe. Only then will we be able to fully thrive as a species and progress.
Moreover, there's no surviving death either: what does it matter to Mozart that he's one of the greatest living composers now, when he died before he turned 36, suffering in pain and spending most of his life in poverty; buried in a common grave with almost no one attending the funeral. He's dead, so he can't really know anything that happened later on, can he?
You're missing the point completely...
Of course Mozart died young, poor and sick. Quite probably, he did not have a very happy life. But just stop and think about the happiness and sense of joy he's brought to millions of people around the world with his music. He has been moving hearts and providing human beings with a sense of bliss like few individuals have done in history.
Humans are social beings. Mozart is eternal, for his legacy lives on in our societies and has an influence on them.
Which brings me back to the first point. That is, I say, the duty that we humans have in this world, and what gives our life a sense of purpose. Make your best to make this world and this society a better place. "Act so, that you maximise the well-being of the highest possible number of conscious creatures."
Achieve something, don't just pass by.