If you are worrying about leave before starting is the army right for you? More importantly are you right for the army. It was much better when leave was a privilege, not a right; then you were so happy when it wasn't cancelled within two hours of starting it!!!
One thing you will learn about leave in the Army is that it can change at any time. Book the holiday and get holiday insurance throughout your whole career as leave can and will be cancelled even when holidays are booked well in advance.