I regular go on long distance multi-day backpacking trips, and have experimented a fair bit with nutrition.
Walking burns about 100 calories per mile on level ground (and more than that if you are carrying a pack or if there is a change in elevation). So those kind of distances could burn 2000-4000 calories, plus add in metabolism of 1500 calories, and you are looking at a very large total burn.
The key problem is that the body holds about 2000 calories in usable energy reserves, and the body can convert fat to energy only slowly. So potentially, you might not have enough energy to get through a single day, much less if you deplete your reserves over multiple days.
The key to the solution is not so much WHAT you eat, but how much. With those kinds of distances, you need to be consuming 3500-4500 calories per day. Timing also comes into it - it is not much use consuming the bulk of those calories in a massive dinner - you need them to be available during the day. You need to start the day with a pretty good breakfast - 500-1000 calories, and then continue on from there. Lunch for me starts about an hour after I hit the trail, and continues most of the day. Keep snacking all the time. Basically, you want to keep your gas tanks topped off throughout the day, rather than trying to refill them when they are nearly empty.
As a VERY rough guide to digestion times: * simple carbs take 20 minutes for energy to start becoming available * complex carbs take 2 hours * fats take 5 hours * protein takes 7 hours
From this, I tend to try to have my main meals higher in fats and protein, but snacks through the day tend to be more complex carbs. I try to avoid simple carbs as they spike and then crash blood sugars too fast, but I do keep an emergency stash of candy and the like if I feel like I am running out of energy - the quick hit can refill the tank and keep me going for a while until some of the other energy sources can become available again.
Many of the resources on this are devoted to marathon runners and the like, who expend a lot of calories in a short space of time. Hence the emphasis on gummy bears, sports drinks and the like. Personally, I don't think this advice translates as well to longer slower activities like walking.
I would look into snacks and drinks (like sports drinks - you can make your own!) to eat while you walk too. And make sure you do what feels good to your body. One of my friends in college and I would go running. I would always have to eat after, because if I ate before, I'd throw it up.
I would recommend experimenting during your training walks to see how you feel, both before and during. I would need a big breakfast before walking 20 miles, but others might need something lighter. Also try different things during your run. Refuling products like Gu or sport beans are one option, as well as small snacks like dried fruit or even gummy bears or jelly beans. Just don't wait until the walk begins to figure out what works best for you.
You'll find different responses to this question because all of our bodies handle food differently. Some can eat a full breakfast and then go out and run 10 miles, others are doing good to gulp down a glass of milk before heading out. The best thing for you is to experiment for yourself. Try a simple meal and see if that holds you. That much walking might prompt a snack for fuel so carry some nuts or fruit with you. See how you do and adjust accordingly.
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