The walking and running metaphor is not the best to describe the situation anyway. It is more like having your parents drive you around before you get your license to drive and then learning how to drive. Your parents did everything for you, so you might know what it is like to drive, you have never done it. You could probably imagine yourself doing it, but not all of it. When you learn to drive, you quickly realize that it is not simply push pedal and turn wheel. There are other factors you have to learn. The whole fact that someone did something for you does not give you the progresses ability to do something.
In other words, while D&D may be good for beginners, it really does not help you to go into GML. If you were a master of D&D, you would still have to learn how to set up GML. Not only that, but GML offers more than D&D does, so even if you know all of D&D, you are still missing out on a lot of stuff.