Writing and speaking are quite different. When you're writing, you have limitless chances to keep re-reading and editing what you've written. Public speaking is more about being able to think fast at feet. It also requires more confidence, of course.
I love writing, and I also like public speaking but I'm not so good at it. Even when I receive a good feedback, most of the times just after finishing I feel, "darn! I could've made my speech SO much more influential and effective by adding such and such, or explaining with such and such example", and so on.
The point is, because you're a good writer, there's no doubt about the fact that you KNOW what you want to say. You're full of excellent ideas to share with the public - and that's definitely a big advantage (and starting point) in public speaking. The challenge, however, is being able to think quick when you're standing in front of people, and saying just "what" you want to, "when" you need to.
I'll also share this: In my personal experience, when I was speaking, I used to start focusing too much on people's reactions, their faces, and so on. It hindered my ability to think fast. When I'm writing, I'm solely focusing on the idea I'm writing about, and can obviously thus produce good quality content - that's the significant aspect that differs the two.
Anyway, it's very good you're taking this step. It's GREAT, in fact. Public speaking is a *very* learnable skill, and with solid writing, you already have a very good foundation of ideas.
All you need is CONFIDENCE. When you're speaking, don't think about what people might think - just divert your attention to your ideas and feel excited for the opportunity to be able to share your excellent ideas with public who will appreciate them - affirmations can cause a huge impact. Or just imagine you're sharing your ideas with your best friend, how will you explain it to him/her? Speak like that, but your face must reflect confidence, and show that you know very, very well just what you're talking about. Being doubtful is where most people go wrong.
All the best.