Putting it on paper.

Translation of ideas from your mind to the paper is more difficult then I anticipated its not the idea thats hard but an idea that has weight. Something you can carry on for a few pages if your writing a childrens book but how much weight does an idea need to write a novel? Or am I just overcomplicating things by putting so much pressure on one single idea to get me through an entire 300-500 pages. But I mean it seems that all other writers, the ones that write something of woth have an AHA! sort of moment. What does it take to come up with an idea like that? Talent, Formal training?