As you brood, you will be in good historical company. Abraham Lincoln was known to brood on a speech for days or weeks. He carried little notes to himself in his hat. Eventually he arranged these jottings in order, wrote, revised, and shaped his speeches. But up until that last moment, he pondered and polished. On the Sunday before he was to deliver the speech dedicating the Gettysburg cemetery, he told a friend that the speech wasn't exactly finished. "I have written it over two or three times," he said, "and I shall have to give it another lick before I am satisfied."
The night before, he closeted himself away from the crowds and practiced his speech. He worked on it all night and was still absorbed in thought as he rode to the cemetery. When the moment came, he delivered the nation's most celebrated 266 words in less than five minutes.
After you've applied the Lincoln method and let your topic simmer in your mind, your next step is to actually prepare your talk, step by step. The order of the steps is also important, because it addresses your concerns in the order they arise. By themselves, the steps are easy to tackle. They take the daunting task of doing a speech from scratch and make it manageable, even fun.