You ask a good question and I can’t answer it. I have used multiple exercise science references that seem to agrees on the 100,000kcal number for total fat stores in endurance trained athletes.
But, let’s start with some areas we do know:
1KG of fat contains 9,000kcal because 1gm of fat contains 9kcal of energy. The water you mention contains no energy so should not be included in your calculation.
At 5.6% of 77kg Steve was carrying 4.3kg of adipose fat. This fat contained almost 39,000kcal of energy.
We also know that well endurance trained athletes can store much more energy as intramuscular fat than non trained people. I cannot find sources that show what this amount is but even the leanest marathon runners are said to carry this 100,000kcal of fat. I have always assumed that this must be in the form of increased intramuscular fat.
14 days x 2000kcal = 28,000 kcal used. I’m not any kind of authority on this subject but can tell you that you will loose a lot of body mass.
I suggest doing shorter trips with similar energy balance and seeing how much mass you lose.