I started writing short stories when I was in high school. I never thought I’d be able to write a novel. I couldn’t imagine writing that many pages on one story idea. But I did.
One thing I will advise is not to force it. Don’t think of it as writing a 300+ page novel. That becomes too overwhelming. You may feel pressured to stretch your scenes or descriptions into more than they should be. Just think of it as writing a story with however many pages it takes to get the story out. Let your characters lead you through the development of the story. Sometimes they take you down an unexpected path.
As I wrote my story, I let my characters define themselves and make their own decisions. They took me places I didn’t expect to go. This is why I have a general idea of what my story will be from beginning to end when I start writing, but I also let my story develop as I go. There were scenes that I had planned to write into the story, but they just didn’t fit with the road my characters were taking. I had to let those scenes go - which was very hard at times. But you have to be true to your characters and their story.
Writing this way makes it even more interesting. As you write each scene, you wonder what your characters will do next or what will happen next. I could honestly tell anyone who asked about the story I was writing that I didn’t know what was going to happen or how it would end. I had an idea, but I knew things could and would change with each scene I wrote.
It may take a few rewrites before that short story becomes a novel, but that’s ok. Don’t feel pressured to get all those pages written the first time around.