If only 3 out of 24 hours of your time are better than unconsciousness in average (including weekends and holidays), and expect that to stay the case until just before you get senile, you're doing something wrong. (There are plenty of exceptions, especially in the Third World, but few of them are hanging out on LW.)
What's the point of earning all that money if you don't have the time to spend it?
The question is - am I doing enough exercise?
I intend to provide a worked example for you to work alongside with your own calculations and decide if you should increase or decrease your exercise.
The benefits of physical activity are various and this calculation can be done for one or all of them; some of them include: