A "long story" is just a short story that refused to end because you kept asking "And then what happened?" one more time. So do that. Again and again.
That sentence is your seed. Everything grows from it. Don't panic. You're not writing all at once. You're filling four boxes. how to f5
Now go f5 your imagination and write one paragraph. That paragraph will become a page. That page will become twenty. You're ready. A "long story" is just a short story