A) General Purpose: To persuade – This topic doesn't have to be very specific, as it is more of a small introduction of the theme that is going to be discussed. The "general purpose" gives you an idea of what is going to be expected or suggested next.
B) Specific Purpose: You should walk 30 minutes a day to improve your health – Now they give you more details and explain you exactly what you must do and why (and that's why we call it an "specific purpose").
C) Central Idea: To persuade listeners to walk 30 minutes a day – This topic talks about the general purpose of the subject or theme chosen, in order to clarify what exaclty has to be persuaded. However, it doesn’t tell you why you have to do this, since that’s the function of the “specific purpose”.