The author is describing the “dark” and “lovely” as death. They have the urge to go to the “dark” and “lovely” woods to follow death but the next line “promises to keep” stops them from doing so because they feel obligated in staying alive and finishing their goals. So I’m conclusion the author has so many responsibilities and obligations to accomplish and they still have “miles” to go. It’s a way of saying for us to not stop our goals or dreams and forget about he temptation of “sleep” Many people would relate to that because many people feel obligated in staying.