I would prepare for the adventure by getting in the best shape I could (cardio and strength), eating well and getting plenty of sleep.

When I'm physically fit, I hardly ever get sick. In the unlikely event I do get sick, I can recover quickly. Even if my body is used to a high calorie intake, my body could adjust to a sudden loss of food if need be. In contrast, when I'm fat or out of shape, I get sick quickly, can't hike far, can't deal with stress easily, am more irritable, can't think as well, etc.

I think about my brother-in-law who is a 40-year-old, round, tub of lard. He's completely non-functional if he doesn't get his daily dose of multiple servings of sugary drinks and desserts. He turns into a big baby if each meal doesn't have something that's highly fatty and sugary. He's totally out of shape, and has no plans or motivation to get in shape at all. I have no doubt that he would not be able to last more than 6 hours in the adventure mentioned in the original post.