The key to extempore speaking is that the words spoken are chosen as we speak them, and what is delivered is a stream of consciousness that is fluent, erudite and articulate, while being unscripted.
Effective extempore speaking always has the purpose of the presentation and the impact on the audience at its heart. In an extempore/impromptu speaking, it is important to speak to the purpose and to consider carefully what effect you want to have on your audience, then the notion that it is possible to simply choose the right words at the point of delivery, without any kind of preparation, is at best naive and at worst disastrous.
Even the few who are capable of it will admit to significant preparation when the stakes are high.
Not only that, you have to consider how long to speak for. Effective speakers, in their preparation, also consider the audience at all times. They ensure that they speak to the audience they have in front of them, rather than the audience they would like to have in front of them.
It will be good to practice with variety of topics to be successful at Extempore Speech.