Everyone is born with a certain genetic makeup. Each of us is also molded and shaped by their his life experiences. How she was raised, how she was treated, who her friends were; these all contribute to building the person that you see today.
These two factors play a large part in determining how hard a person is willing to push themselves in the task at hand. But they are not the only factors. I believe with all my heart that God created humans with an incredible capacity to improve. Christians see this every day in the way the Lord is transforming their lives. God changes our desires and gives us guidance and direction. But He leaves part of the work for us. We have to make the change.
We are more like children than puppets. He shows us the way, but it is up to us to take the step. We must choose to improve, to take the next step of faith, to become more like Him. Physical fitness is no different.
While there is limitless contradictory information out there, most people know the basic actions it takes to get healthy. Develop an effective workout routine, eat the right foods and the right amounts of them, and get enough rest. Knowledge is not typically the problem. Action is.
Genetics and experiences do not have to determine your destination. You were dealt a hand, but there are many ways that you can play it. Choose wisely and make your own experience. Push for one more rep, one more lap, 5 more pounds, one less Coke. Be relentless.