Some say I'm an overnight success. Well, that was a very long night that lasted about 10 years. But while I do, of course, now feel the pressure having had books that have been very successful, I just know I have to concentrate on writing for myself. I can't worry about genres or markets or what might be commercial or not. That never works.

The speaker is saying that even though it might look like they achieved success quickly, it was actually a long process that they worked on for 10 years. They feel pressure from the success of their books, but they know that they have to write for themselves and not worry about what might be popular or what genres might be successful. Writing for fame or money does not usually produce good results.