On the surface, this looks like very simple advice about the value of practice. A more encouraging and supportive version of "practice makes perfect." However, I think there is more going on here.

Marie-Louise von Franz in her book, Creation Myths, discusses the creative process in language very similar to Glass, albeit in a much less supportive and more sarcastic tone. 

"People in a pre-creative stage are inflated; they are identical with their inner conception and filled with its glory and beauty and its load of energy. But generally when the work is finished, instead of being happy they feel a bit deflated and sad...It is one reason why some people never get down to do something creative, especially men of the puer eternus type, also women of the puella eterna type. They never step down to do something creative because they feel vaguely, or foresee intuitively, that if they produce the thing they have in mind it will be much less good than what they conceive inwardly. They do not want to go through this process of creative deflation but prefer to remain all their lifetime would-be artists, or would-be geniuses, on a very great scale rather than the producer of a humble product on a very much lower scale."

The valuable point here, I believe, is the equation of  the "pre-creative stage" -- Glass' beginners -- with inflation, and that of humility and deflation with true creation.  This makes sense from the standpoint of Jungian psychology, as the true source of creativity is understood to be the unconscious, just that part of the psyche least accessible to ego.

The ego's need to be the "would-be artist" must be relinquished so that the creative unconscious can bring forth it's gifts. For Jung, creativity involves a lowering of consciousness, which he describes as "an empty stillness which produces creative work."

My own experience is that sitting down to write or engage in some other kind of creative work always starts with a struggle to get my thoughts and desires--that is, myself--out of the way. Sometimes this can be a real battle as I keep trying to force the issue, trying to come up with some clever line to begin with, each one more embarrassing than the last.

Almost inevitably I feel like quitting. Sometimes I do. Sometimes, however, if I stick with it, I enter into what I can only describe as a state of alert listening. I am tempted to call it an almost Satori-like state, though perhaps that's a bit inflated. Out this listening a few words emerge on the periphery of my awareness and if I can follow this faint trace, something good appears. It is a challenging process, but it can be an exhilarating experience.

Allowing the deflation, allowing the lowering of consciousness--the empty stillness--is hard work. I am often surprised at what resistance I put up again and again to the creative process. The ego doesn't like to cede its place.

Ira Glass' advice to "do a huge volume of work" is as much about affecting a transformation in the individual as it is about improving one's work. This makes a lot of intuitive sense to me. It is an alchemical process: The transformation of the material being worked upon produces a transformation in the one performing the work. Perhaps in this way, we become what we do.  

"Throw yourself like seed," says the poet Miguel de Unamuno. This is his image for the creative deflation, which is really a deflation of the ego. It is both a call to throw one's self-centeredness away as well as to throw oneself into creative work. He reminds us, in words very similar at times to Glass, that:

to live is to work, and the only thing which lasts 
is the work; start then, turn to the work.

Healing for de Unamuno is to move forward into creative work, to become your work. Healing does not come from looking back and you should  "not let the past weigh down your motion."

From this point of view, our life task is not to figure out who we were or who we are, but to actively participate in our own becoming. And the path to this becoming is work, for

from your work you will be able one day to gather yourself.