Earlier today I followed a link from LinkedIn to a Fox News health article on reasons why you're not as lean as you want to be. While it's not one of my normal sources for health-related information, that article articulates well something I've held to be true for quite a while now:

You Believe the Myth of Discipline

There is no such thing as discipline, all we have is love. Either you love being lean and ripped, or you love burgers, fries and donuts. Stop making excuses about having a “lack of discipline” and man up -- what do you love the most?

Now, I'm not focusing on getting "lean and ripped" necessarily, but I think this idea holds true to fitness in general.

When I was very overweight it wasn't because I had less discipline then than I do now. It was because my primary desire was to eat whatever I felt like and to relax in my off-hours. So, while I didn't necessarily want an overweight body and generally poor fitness, it was the cost of putting those things first.

A change of mind and heart was required to get fit. I had all the knowledge and resources necessary to get fit but it couldn't happen until being fit became a desire greater than my desire for food & relaxation. I didn't get some infusion of discipline which allowed me to lose weight, rather I changed my mind with regards to what I really wanted.

Ultimately, I believe this is the most important aspect of getting fit; the change of mind that puts being fit at the top of your desires.