I have done many centuries this year including today. Average of 18 MPH is hard work. Perhaps you should think of 16 MPH average and a significant rest period at 50 miles?
Please do not get me wrong. There is nothing wrong if someone does 100 miles in 5 hours. But it takes a bit of conditioning to do that.
That is on top of all the other good advise above. Nutrition and hydration.
My century today was on limestone at 17.5 MPH average. It was murder and I was dead tired and had trouble driving home for 45 minutes.