I do this. I exercise, chill out, and then write fiction for a few hours before I go to work.

I wouldn't say I'm an overachiever but I do work at a fairly high-pressure, competitive BigCorp. Haven't been fired yet, and my performance reviews are fine. The truth is that if somebody else owns 100% of your highest level creative output, he owns your soul too, and I'm just not down for that.

It does mean making sacrifices in other areas, but for me personally the alternatives are worse. Obviously not everybody has the luxury of so much (theoretical) free time.