How to Train For and Run Your First 100 at the Umstead 100 (Blake Norwood).
Also see these resources:
Here's a calculator to help you gauge when you'll start each lap. You'll have to adjust the times if you've planned a nap in the middle.
The 50 mile split ratio is a measure of how much slower you'll run the second half of the race compared to the first. Historically, runners take about 30% longer, hence, the 1.3 default in the calculator.