The cultivation of skill takes time and requires patience. Your writing will not get better until you have written many, many pages.
Journaling is a good way to accomplish this.
When we journal, we let the writing flow. Whether for a paragraph or a page, the pen moves along. Using a pen and paper is recommended. Again, the computer or tablet is just too distracting. Your mind is reminded of where else it could be. A pen and paper focuses that creative energy.
Journaling is training for the mind. It is training for the hand. That is the way to build stamina.
No. Journaling will not make you better at plotting or character development. Journaling will not help with worldbuilding — though it will help you with brainstorming.
Journaling will help you get used to the act of writing; it is just an act, after all. The more it is repeated, the better you will become.
The mind enters into a certain place when this practice is underway. It becomes accustomed to producing words. They may not be the polished words of a final draft, but they are the beginning.
All works begin in that primal state. Become comfortable with that state through journaling and witness your abilities as a writer increase over time. And that is the kicker: Either way, it will take time.