"We are what we repeatedly do. Excellence, then, is not an act, but a habit." - Aristotle
If you want to know why your life is where it is today, look at your past habits. If you want to know where your life will be 10 years from today, look at your current habits.
If you have a habit of eating junk food, it's going to affect your health. If you have a habit of procrastinating, it's going to affect your level of success. The difference between those who are successful in life and those who are failures in life comes down to what they do on a daily basis.
Is it hard to go to the gym just once? Of course not. Is it hard to do it consistently for the rest of your life? It will be at first, but once that action becomes a habit, it's easy. Ask anyone who's working out for years if it's a struggle for them to hit the gym and they'll tell you it's like brushing their teeth, it's automatic.
The point is simple. If you want your life to change, look at your habits. Create the necessary ones and eliminate the counter productive ones. Once you've done that, let time do the rest.