I concur with Tao- you don't need to be a theist to have morality. Ethics and empathy are part of being a social creature, and a person can learn from nature itself what it is to live a good life. It's common sense that humans are social and need the group to survive and propagate- and more than this, to be happy. All living beings also need the larger natural system as a whole in order to ensure life for future generations. In supporting the social group and the natural world, we can find all the essential ethics for a good life- caring for others, being sustainable, and so on.
The problem with human beings is that we are so socially adapted that we can actually perpetuate maladaptive cultural "rules" that are detrimental to real ethics and also to our long-term survival. What worked through common sense when we were hunter-gatherers, living in small groups and seeing the results of our decisions directly doesn't work in a global capitalist and overpopulated situation.
The way I see it, as a panentheist, is that God is all over the place- in us and all around us. Our opportunity to learn harmonious action with the whole, to have a good and happy life, and to connect to that "Something More" if we so desire is all around us and in every day and every being and every moment. So I figure that if someone doesn't want to "believe in God" it doesn't mean s/he won't be communing with God, at least my sense of what God is.
That said, of course one can choose disharmony or be too lazy to care... which is not only unethical and against what I would say is "Divine Order" but also is just plain stupid. As we can see with environmental deterioration and widespread poverty and violence, the results of this speak for themselves. Again, it's pretty much common sense to me.
As for myth, in the animist traditions, these stories encoded information useful for survival and long-term sustainability much of the time. Information beyond things like the golden rule that were specific to the ecology of a group's particular place. As such, they are/were useful ways to memorize and hold to a bulk of information. They are great learning tools for many humans, as it works with how our brains function, and they have the bonus of making us emotionally invested, thereby ensuring our commitment to proper behaviors. While I find the value of the world's religions in general ethics to hold true, we have largely lost the value of religion for practical matters such as health, economics, and sustainability. I find that to be a shame, because it's apparent that, on the whole, we haven't filled in the void with something else. Of course science has lots to say, but without getting the billions of fellow global citizens on board with proper behavior, we continue to screw over each other and our planet.