Everything in creation has its limits. From the most basic elements to the highest of all beings - man, all have something that limits growth or action. This fact does not detract from the greatness of this universe nor the mind boggling complexities within it. Humans are the one phenomena that may take exception to this truth. As thinking beings, humans want more for themselves, be it physical or spiritual. Man does not want to be held back from his desires or dreams.
Claiming that the creator has placed limits on man's potential is just an excuse because man can reach most of what he wants. It is truly remarkable what people can accomplish when real commitment and perseverance are put into play. Most of the limits man has are self imposed.
As a fitness trainer I encounter this often. The clients who put the time and sweat into their sessions absolutely see results. Because of the effort they expend, they get stronger, increase their mobility and most importantly, feel better and are happier. These people come to their session even though they are tired. They come even though there is a distraction that day, or some other excuse to cancel.
I too have this challenge in my own workouts and running. I know well that I will have to work hard to get myself to where I want to be with my fitness in general and specifically with my running. I know that I have to get the miles behind me or I wont reach my goals. So I push myself and I am happier for it.
So stop making excuses. Push yourself. Reach your goals. You are human and capable of great things!