Andy,
my opinion is coming from the point of view of an endurance athlete, I know very little about type 2 diabetes I'm afraid.
I would look for a mix of simple and complex carbs, and also over that distance/time a bit of protein as well. You'll not be exercising at high intensity, so after about 90min-2hrs you should be going into more of a fat-burning regime anyway, but you still need to keep some circulating blood sugar to make that mechanism work. To be honest most people have plenty of fat reserves to be able to walk that distance without eating anything, they'll just be happier if they do.
My favourites through experience are NAKD bars (dates, nuts, oats, raisins), 9-bars (Nuts, seeds, honey) and peanut butter on wholemeal toast or a bagel. I'm not sure how you tolerate malt loaf, but that's always another good one as it has a good supply of carbs, but low sugars so it's absorbed a bit more slowly again.
The other thing you could do is make your own trail mix, but go heavier on the nuts and cut back on the raisins and dried fruit.
And don't forget drink
Dave