The following pieces of information are both true but may seem to directly conflict with each other.
To succeed as a professional writer, you should be flexible and try many types of writing.
To succeed as a professional writer, you should find a niche where you are paid well and then stay there, wracking up payments and points by writing more in your niche than everyone outside of that specialty – sometimes more than everyone else put together (sad, but true except for the happy workaholics among us).
Amateur writing can be the same way, but it depends on how you developed. Through about the 3rd year of university I was still getting the odd English literature course. For those who wish to scour the past looking for precursors, I wrote a one act play in the 2nd grade outside of school, kept a diary, and did get published in the high school magazine when I graduated.
Most of the professional writing I have done has involved parameters unlike those in the literature courses and I have never been paid for writing in iambic pentameter but think it would be truly great (not being sarcastic) if I had been paid to do that by now.