Visually judging distance without an aid can be difficult. Some learn, some never do. I sometimes believe that some people that are consistently good have mental range finder in their head

I have seen some shooters do fair while on the ground and in an area that does not have much visual obstruction and contrast. Get them in the fields and, especially, in a tree, the same people cannot even come close no matter how long they practice and are exposed to range-estimating situations.

I have never had a problem with visually estimating distance. If you ask me to verbally state the estimated distance, I can and I am usually very close or right on. However, when I am shooting I do not mentally register the distance in figures. I just feel the distance. I know that (feeling it) sounds strange, even untrue. Nonetheless, it is true in my case.

If you know that you just cannot estimate distance, and do not believe you will develop the skill, set up visual distance markers in your zones. For insurance, I sometimes use markers when the area I am in is hilly and overly heavy in contrasting objects, color, and light.