Things to keep in mind:
* With a typical trail race, people hike the uphills. When you’re on a flat course, you don’t have uphill hike breaks. I know a lot of people who succeed at a flat 100 mile by intentionally implementing a run/walk ratio.
* Even a flat trail race still has rocks and roots. Once you hit nightfall, the terrain and general fatigue will likely cause you to slow down. I know some people who run all night. I know some who powerhike all night. If you don’t do a run/walk (like mentioned above), simply walking more at night will slow down your average.
* Don’t forget that 100 mile aid stations will include real food (that is hard to eat while running, so you’ll need to walk) and the opportunity to sit down and relax for a few minutes.