Writing something that will be spoken - or even performed - is different than writing that will be read. People do not speak in the same way they write (certain Aaron Sorkin characters excepted),
Back in the planning stage, you decided what you wanted people to walk away with from your talk. Now as you write, you can chart your path to get people to that point. What pre-existing ideas might you need to change their minds about? Which building blocks of knowledge will support the conclusion you want them to come to?
Put yourself in the place of a sceptical audience member - what are the most obvious rebuttals to your argument? Try to address those in your writing so you present a stronger case. Some questions to get you thinking:
Listening to people talk takes a lot of concentration. Give people some mental breaks by using sections, summaries, recaps, and pauses.
Starting with a bulleted list of the points you would like to make can help in two ways. It gives you an overview of your whole argument, which you can then strengthen and rework, and it makes it easier to expand one outline point at a time without feeling overwhelmed.
Reading out a speech at one volume, and in one rhythm, is a great way to bore people. Look for places you can insert some variation. That could be a surprising chart or image, a change in speed of delivery, a contrarian idea - anything that will recapture the attention of people who might be 3 talks into a session.
Sharing information is important; stories will make that information stick. Stories get people interested, and are your chance to connect your personal experience to the message you’re trying to share. What have you seen, done, or failed to do that puts you in a better position to make the claims you are making? Share those stories and build your credibility.
A good opening section will build your confidence, and will get people engaged enough to listen. Don’t waste those first few moments with a dull recitation of your job or the title of the talk. Say something, or show something, that grabs their attention.
Similarly, your closing points should leave people with something to think about. It’s your last chance to solidify your message, and to leave people excited. Make the most of it!