The body is weird as you know, it reacts differently for everyone, for me it was eating, it rejected food from day 2 and 3 of my epic but it soon found it self sorting itself out. My body got stronger from Day 4 onwards.
If I did more than 7 days of ultras around 10 days the body would think it was norm. You would have to expect the body to have one shit day every 10 days or so though after that so facturing that in will help.
It's all about the mind, I know a few who have failed similar goals because they let their mind take over way too much. The body can do more than we normally think