Can you succeed living a balanced life?

Growing up parents and teachers harp on the importance of living a balanced life. Parents want their kids to excel academically, socially, athletically, and musically. College admissions won't shut up about how well-rounded they want their students to be. But I've noticed something: no one seems to accomplish anything living this way. Success (as defined by an entrepreneur) seems only to be found at the extremes.

This subject matters to me because I'm currently working a "real" job and trying to get a startup ready for the YC winter session. This amounts to ~16 hours of coding every day, so my life is anything but balanced. One of the things that keeps me going is the above observation. Anyone else believe this to be true?