The old saying is very true but the one caveat with motorsport is there is never enough time to prepare sufficiently.
The common held belief is it takes 10,000 hours of practice to become a maestro at any skilled activity. If you tested solidly 8 hours a day, 5 days a week that would take you 5 years. As a racing driver you do not have that opportunity. You can’t test solidly for 8 hours a day, you need to refuel, change tyres, rebuild the worn out car, train for the marathon 8 hour stint every day.
As such you can never be fully prepared for any event or opportunity but focus on the important things so you can be as prepared as possible.
Have you driven the track before? No? Watch YouTube videos and drive simulators. Have you driven that car before? No? Research the car, ask people who have driven it for their advice. Have you used those tyres before? No? Get some advice.
In short, locate the main weak point in your knowledge for the upcoming opportunity and focus on it.