I've been a "lurker" on this thread, but I've followed it with considerable interest. For most people who have not been certified early on as potentially world-class (by virtue of, for example, a 3:53 mile in high school), the decision to dedicate a large chunk of one's life to the pursuit of competitive running goals should not be made lightly. That may be especially true in the U.S. and other societies where intelligent, hard-working people have many options in life. In economic terms, the "opportunity costs" of running are greater where there are attractive alternatives available. In a society where there are only poor alternatives, the opportunity costs are much lower. As a singer-songwriter observed, "When you've got nothing, you've got nothing to lose."
If you are tempted to chuck your job and everything else to become a super-hard-core runner, consider the possibility that the temptation does not arise solely out of your desire to be a good runner, but rather also arises out of your dissatisfaction with other aspects of your life. In a society where people have lots of options, there may be better options than casting aside everything else to pursue your goals in running.
Although this message may sound critical of those who decide to take the plunge, it's not meant to be. I can't honestly say that the people I've known who have been full-time or "underemployed" runners have, in the long term, been disproportionately unhappy about the choices that they've made in life. I do believe, however, that some would have been better off if they had dealt with their dissatisfactions in other ways.
I ran sub-2:20 marathons while working brutally hard hours in a very stressful occupation. It can certainly be done, even by runners of rather modest talent. But there are costs to leading a life that is dominated by concerns about managing time and managing fatigue. My own life during those years was very monastic, and almost every day seemed like a battle to keep pushing forward. I can't say that I regret those years, but I probably wouldn't recommend that life to most people.
In my opinion, these are really hard choices, and different people will legitimately choose different paths.