While time is the great equalizer of life, the amount of energy you have and continually cultivate will determine how engaged you can be in the activities you are involved in.
The ultimate assessment is: How much energy do you have in any given moment or day?
Inspiring Resources on Energy Management:
Manage your energy not your time article (Tony Schwartz)
Manage Energy, Not Time video (Tony Schwartz)
Book: The Power of Full Engagement (Jim Loehr and Tony Schwartz)
Energy Management (Matthew Kelly)