I've had some intense workouts, and have gone from 30+ lbs gain to 30- lbs weight loss from a lot of cardio (losing fat), and a lot of resistance training (adding muscle).. I think little has to do with actual exercise itself, but rather where you're currently at in your life at that point n' time.
In other words.. if you're really pumped up and excited about progress you've made in some other aspect of your life (like say physically via weight loss), that inspiration and motivation will carry on over through other parts of your life as well.
In the end, being physically fit shouldn't be a vehicle to solely increase work ethic, creativity etc.. but rather just a fundamental part of your life to keep yourself healthy. Work ethic/creativity/all other areas which require progress and focus should deserve direct attention, not attention on behalf of improvement from some other area of your life.
edit: i'm strongly buzzed, disregard if it makes no sense.