CreakyPete wrote: and afterwards fat is turned into glycogen to replace the losses.
Not strictly true. Fat is not a major source of glucose or glycogen. Fatty acids released from fat stores are burned directly by the muscles and so can ketones produced during fasting, this is not a rapid reaction so that high intensity exercise is fairly impossible once the glycogen stores are used up (though this takes days if not exercising hard).
Muscle fatigue (hitting the wall) is not the result of the depletion of glycogen reserves. Nor is muscle fatigue caused by the accumulation of lactic acid. Muscle fatigue is caused by the decrease in pH caused by the conversion of glucose into lactic acid. The pH of muscle cells may fall to 6.4 during intense muscular activity. As the pH decreases, glycolysis slows, slowing down the production of ATP, producing fatigue.
During fasting or during excessive activity, muscle tissue is degraded into amino acids and the carbon skeletons are used for fuel. Muscle tissues lack the enzymes to convert protein into glucose. Instead, the amino groups are converted to form alanine. Alanine is released by the muscle tissue into the blood stream where is absorbed by the liver, and converted into glucose by gluconeogenesis.
Running a marathon requires the mobilization of several metabolic fuels and requires the cooperation of the liver, muscle and adipose tissue to mobilize the energy so that it can be tapped for the long run. Trained marathon runners have increased glycogen stores and burn equal amounts of fatty acids and glycogen during the run. In order to do this, the blood sugar level must be low to produce a highglucagon/insulin ratio. This high ratio mobilizes fatty acids from adipose tissue. These fatty acids are absorbed by the muscles, and oxidized completely into CO2. Enough glycogen is spared this way, to sprint towards the finish line.