Breaking Into the Writing Business: Challenges

What do you think are some of the most challenging issues when it comes to writing your book? Is it writer's block, seeking the right agent, formatting the book itself , submitting a great query letter, getting published, ect,.?
      So far, the main issue that I have encountered has been in marketing the book. Opening a fan page or two to promote yourself is easy, but how can one increase their reader audience and promote their book? Any suggestions are welcomed...