A hunter, wakes up one fine morning, takes his gun (or bow/arrows) and goes for hunting. If he is lucky, he may shoot an animal and bring home some food. If he is unlucky, he may starve for weeks together.
Similarly, toastmasters provides a structured program to improve communication skills. Toastmasters provides an opportunity to nourish & constantly re-enforce your communication skills.
One of my toastmasters friends said, "You cannot be a seasoned speaker by just being a seasonal speaker". You should be constantly practicing in front of people to deliver speeches. That is the only way by which you can be a successful public speaker.
You cannot swallow a "magic pill" before going to bed and get up the next day morning as a "seasoned public speaker". The law of harvest applies even for sharpening your communication skills. It takes time. But, if you are constantly working on it, you'll be a professional soon. Start ploughing the field today, to reap the benefits tomorrow.