The higher up you get in an organization, the more you need to focus on what the company needs from you, not on what you want to do. While management experts advise doing what you are best at, great leaders do those things that only they can do. Figure out what functions only you can perform. These may be tasks that are unique to your role, such as meeting with a key client or calling a top official. Or they may be strengths that the organization lacks, such as solid marketing skills. Gauge how you can be most useful and focus your time and effort there. Then delegate the rest.
Source: "Managing Yourself: Extreme Productivity" by Robert C. Pozen.