Try to meet as many authors, agents, and editors as you can.
I was pretty much grown-up by the time I attended school in Britain - or as grown-up as I'll ever get.
Now I have to motivate myself much more than I had previously.
How long it takes to write a book depends on its length.
I've learned that I get blocked when my subconscious mind is telling me that I've taken the work in a wrong direction, and that once I start listening to what my subconscious is trying to tell me, I can work out the problem and get moving again.
That's why editors and publishers will never be obsolete: a reader wants someone with taste and authority to point them in the direction of the good stuff, and to keep the awful stuff away from their door.