Here’s a (Biochemistry, 5th Edition. great paper on what happens during a short and a long fast. Berg JM, Tymoczko JL, Stryer L.).
The paper covers the daily feast/fast cycle of the regular American diet. It also (Biochemistry, 5th Edition. covers what happens over a longer fast. Berg JM, Tymoczko JL, Stryer L.) One of the interesting charts relates to a longer fast:
The chart on the left is the Glucose level over a longer fast and the chart on the right is the Ketone Bodies chart. Per the charts the Blood Glucose falls faster than the Ketone Bodies can kick in. That could the reason that the third day of an extended fast is considered to be the toughest. This also explains why it is much easier to enter a fast being already in Ketosis because your Ketone levels are higher.