Are you only a part time manager?
According to one recent survey you probably are. The survey on executive time-use reported that on the average only 47% of the manager’s time goes into managing. About 31% is spent on work that can be competently handled by a junior clerk, and the rest of the time is spent on showing others how to do their jobs.
The survey also revealed that most managers start the day with only a general idea of what has to be done. Then almost at random they take on each event as it comes. As time memory is notoriously faulty you probably feel that none of this applies to you. The best way to find if the cap fits is to keep a daily time log. Do it for a fortnight and then reassess whether you need to base your day on a more exacting timetable.