I've just come back from doing part of the sentiers des bonshommes, a much shorter walk than yours (about 120km but a lot of climb with a backpack)
It's the third such walk I've done and each has produced slightly different problems but basically it is sometimes quite difficult to keep glucose levels high enough.
When on MDI , I certainly had to reduce my basal but didn't really do it by enough, my doctor when I came back told me that I should probably have reduced it by half but that was based on me others may have different requirements.
I now have a pump and 40-50% of basal works best for when I'm actually walking, I also take a reduced bolus at breakfast, no or very little bolus at lunch,(I don't tend to eat that much for lunch, often bread/cheese/tomato. For the evening meal, I sort of play it by ear: depends upon the type of meal/how many carbs and how hard the day has been. There is a potential for hypos during the night if you take too much insulin with that meal.
(all in all on the last trip I was using about 16u of insulin a day compared with 25-26 the previous week and I was eating a lot more carbs than usual.)
During the walk, I used a variety of things from cookies (I tend to eat just a half at a time... share with my husband), dextrose for very low lows . I discovered some gels in Decathlon that were slightly different to the usual sickly sweet stuff being made of raspberry puree and glucose, they had nine point something grams of carb . I found these very useful for warding off hypos when I was in the 4-5 area and had a big climb ahead. I also used some energy bars for this They were chocolate and nut ,were supposed to be mixed low/high g and had about 20g carb. I ate half at a time. I keep the carb content of 'snacks' fairly low as I hate raising my glucose levels too high for exercise but pay the price in that I probably go low more than some people would want or feel safe doing.
It is certainly worth looking in a good sport or cycling shop for a variety of products that can help. Some of them contain a lot of chemicals though, that's why I liked the raspberry puree gels; they seemed more 'natural'
I didn't use much fruit as we weren't near shops and it would have been heavy/difficult to carry but that is always a good option.
You must build up to it, 30 miles a day is a fair distance and not something to be undertaken without a bit of training which should help you decide on strategies that will work for you. Test frequently.
( though you may not need to reduce basal in quite the same way for a one day walk, I've found the effects are cumulative)