It makes a difference having a specific purpose. I’ve been running off and on now for a couple years. But for the first year and a half, I ran simply to keep in shape. I never ran more than 3 miles at a time. And that was very difficult. But for the last 6 months or so, I developed a desire to run the half-marathon. So, I’ve been training (not very seriously though) for it. Now, I can easily run 6 miles. I’ve ran 12 miles or more several times. What made the difference? A purpose. 
What about your life? What are you living for? What is your purpose? Do you have one? 
Or are you simply living because you don’t want the alternative, which is dying. Most people, I’d venture to say, do not know why they are living. They live because they have to, and they don’t want to die. That would be running away from rather than running toward. What an aimless and useless life that would be!
“So I do not run aimlessly; I do not box as one beating the air. But I discipline my body and keep it under control, lest after preaching to others I myself should be disqualified. 1 Corinthian 9:26-27Paul had a definite purpose and calling for his life. And it motivated him and kept him to the task until the day he died. His whole life was devoted to this singular focus. How awesome!
Do you have such a focus? What would be worth dedicating your life to? Find it out. Ask God. Don’t rest until you find it. 
PH