I don't fast hardcore, even on Good Friday. Too many times I've tried with good intentions only to feel my blood glucose fall, my ire rise and the bakery sing its Siren song.
I wake, hours later, covered in powdered sugar and shame. Enough of that.
Better to remember that there are forms and variations of fasting, some of which might be described as restraint or adopting a specialized diet. Consider the hot cross bun, traditionally baked and served as a Good Friday fast food. But it looks like a little panettone to me, and a frosted one at that. But it lacks meat and that's restraint for some.
I look a little farther east for my guidelines, to xerophagy: a diet of bread, water, nuts, fruit (especially dried) and simply prepared vegetables. While associated with Eastern Orthodox monasticism, with its genesis in the desert, there is something about it that reminds me of hiking and far journeying. I suppose because I can imagine a xerophagic diet be easy to carry in a backpack. Take that for what it's worth. Gorp, crackers and water would work. I wouldn't say no to an apple or an undressed salad. Indeed, I might look for these to go with my almond butter sandwich and raisins.
But just enough to get through the day.