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?

>>5
>>3 here,
basically, my whole post says the same thing as your >>2 , as I can see now.