The heart is not just an internal organ it is a muscle. Like most muscles, it can be exercised, it can be made stronger, it can get bigger. The heart’s work is to deliver blood to other muscles in the body, and in most cases the bigger the heart the better capable for work.
I used to be more of a runner, and I preferred longer distances rather than sprints. According to some research, an endurance athlete’s heart can be up to 50 percent bigger than a non-athlete’s heart (source).
There are a number of things necessary for running; the blood-pumping organ doesn’t map out the course or see the mangy dog straying outside his fence. The heart doesn’t take even a step by itself, but it enables the legs and feet to stride and the arms to pump for forward momentum.
When it comes to obeying God, we do need knowledge of His law, and His Word lays out the map. Psalm 119, for example, is full of prayerful praise requesting open eyes (verse 18) and learning (verse 7) and understanding of His precepts (verse 27). But there is another request the singer makes that we would do well to imitate.
I will run in the way of your commandments
when you enlarge my heart! (verse 32)
In one way, our hearts are enlarged as we obey; obedience is a strength exercise for greater obedience. Obedience begets obedience. And also, the Lord Himself will expand and spread and increase our heart size. It may not be the case that you don’t know how to run in the way of His commandments, you are just out of shape. Pray that He would make your heart wide.