How we spend our days is, of course, how we spend our lives.
Dillard wrote this in 1989, in a book about the grinding, unglamorous work of being a writer. She was describing how easy it is to fritter away hours on small distractions and call it a day, then look up and realize years have passed the same way. The line cuts to the center of autonomy because it removes the excuse that your real life is somewhere ahead of you. What you actually do today is not preparation for your life — it is your life.
Reflection
Most of us have one daily habit we know is eating our time but keep anyway. What is that habit, and do you actually want to stop it?