Oh it’s so important, I’m sorry you missed that memo!  My doc told me two years ago that I should elevate my heartrate for 15 minutes a day for the best effect.  I also found that, on days I couldn’t get 15 minute run in, a walk helps too.  Especially considering sunlight is another natural aid for depression. (side tip: exposing yourself to sunlight for 15-30 minutes between 7-10am helps your body set its biological clock and leads to more regular sleep patterns).