Being prepared ahead of time allows you to make better decisions about what you will do, have a quicker response for those who make demands on your time and remain focused on effective actions.
And don’t get sidetracked into agreeing to do something else. If it is related to your not-to-do list, it probably means don’t volunteer to do it. It’s not that you are being miserable. It’s that you know what your priorities are and you are vigorous about accomplishing them.