Novel =/= Short story

So, I've been writing since I was very young.  The problem has never been, "What to write about next" - but more along the lines of keeping up with the last thing I wrote about. 

As in, all I ever seem to get are half stories (the longest being about 20 pages). Any tips for actually writing something substantially longer,  instead of ending up with a short story?

Plan it out first. When you know where the end is and where the landmarks in your plot are, it will be so much easier to fill in the blanks. Once you find the scale of the piece, the events that fill it change from being important story elements that motivate the plot to being one more thing in the book. And since you know the shape of the book, you know what sort of thing will fit and you can just put it there. The plot is there and will happen on its own, see, so you don't have to worry about advancing it. You just fill in the gaps with expository anecdotes that explain motivation, fascinate the reader, and so on. And you describe the plot events in detail, of course. But everything comes from knowing the scale.

DISCLAIMER: I write music, not prose. But my gut tells me this will be helpful to a wordsmith as well. There are probably some (many) writers who can begin at the beginning and wander their way to the end. Since you are not yet one of them, it would behoove you to attempt premeditated structural techniques.