Did Emerald Coast prep back in May, nearly nine months before military retirement & planned to attend a follow up course at the beginning of October. Problem was I interviewed the day of the refresher prep. Point being, don't assume you have time after you hit submit on your app.
Definitely do professional prep, but make sure you live practice with someone before your interview to refresh what you learned. I worked with a buddy to rehearse, but also contacted a local public employment resource office (Phoenix Workforce Connections) & ran through two hours of sample questions with a stranger.
Good luck to all!