I did a 100 miler (well 97.4) with only a bowl of crunchy nut cornflakes and a banana before I set off. I had a pannini and a bit of cake at a cafe at about the halfway stage. It was at about 80 miles that I hit "the wall" and had to resort to my emergency food rations. This consisted of some Tesco trial mix. I immediately felt better. I can seriously recommend the trail mix, it is excellent: it is a mixture of brazil nuts, dried fruit & dried banana.
During the ride I drank approaching 6 litres of water and didn't have a waz all day long. Once I got home, I had a headache, felt sick and had no appetite. People on here told me the reason for all this is that my body had entered "Ketosis". Which basically meant that I had totally exhasted my carb / glycogen supplies and my body had started to convert fat to fatty acids and ketones and used that as an alternative fuel. So basically fat burning. I don't recommend going this far, as I felt bad for about 2 days.
Next time I do a 100 miler, I will have a bit more to eat before I set off.