It’s hard to be an absolutist on such a big question. Everyone has distilled it in their own ways. I will throw mine:
There is no purpose of life. Life is THE purpose. To experience its full brilliance is the ONLY purpose. And only way to experience that is through a healthy mind and healthy body. So do what you can to take care of those.
I've seen this play out badly for some though. As you correctly state, hard to be absolute about such things. For some people, it can escalate to a very egocentric behavior, in turn making live worse for others. One could argue most of us wanting the best living standard all the time is perhaps the main cause for a lot of problems around the world, including global warming.
If you one day wake up inside a box isn't your purpose in life from that moment on to find out how to escape from the box, and if that turns out to be impossible to figure out what the box is made of, etc.?
It seems that many people could enjoy a rich life, but instead they are thinking about that box all the time.
There is no purpose of life. Life is THE purpose. To experience its full brilliance is the ONLY purpose. And only way to experience that is through a healthy mind and healthy body. So do what you can to take care of those.