I am not an expert on this, but my the first thing that comes to mind when I read your post is that the advice you are reading about here on the forum is for people that are already riding 20 or 30 miles and want to do 50 or 100 miles at speeds of 15-20 mph, while you are doing only 3 or 4 miles at much lower speeds.

These are 2 different worlds, it is all about what is the limiting factor. And for you it is definitely your weight that is holding you back, and your cadence just feels like a minor detail to be worried about. So for you the fastest way to be able to go long distances is to loose weight, that will enable you to go longer and faster.