When most people think of what the body needs to cope with running the 100km distance, the only consideration is usually hydration and fueling. However, that’s barely the tip of the proverbial iceberg.
Yes it is true that fueling is important. You have to remember that ultra distances of 50 miles and longer require more and more fat conversion for your energy requirements than a marathon that can be largely completed using long chain glucose polymers.
If you haven’t trained your body to cope with running in a fat adapted state, you will not tap into that fuel system during the race. You will likely bonk as you cannot consume and digest enough carb calories to match your energy output.