The myosin heads of muscle fibres move using the energy produced by converting ATP to ADP. The energy extracted is about 30.5 kJ/mol of ATP.
Muscle tissue has limited ATP storage. Your muscles' primary fuel storage is local Glycogen.
Converting Glucose to ATP
Glycogen is converted to Glucose.
Glycolysis, the Citric Acid Cycle (aka TCA Cycle, Krebs Cycle) and Oxidative Phosphorylation each produce ATP based on the byproducts of the previous process. These three processes produce 39 moles of ATP for each mole of Glycogen.
For more detail, see http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect12.htm
Glycogen storage capacity in man is approximately 15 g/kg body weight.
Glycogen has a molar mass of 666.58 g/mol
For an 80kg man:
Glycogen storage capacity: 15 * 80 = 1200g = 1.8mol
ATP producible aerobically = 1.8 * 39 = 70.2mol
Energy available: 70.2 * 30.5 = 2141.1 kJ = 511 kCal
Using this calculator: http://www.tribology-abc.com/calculators/cycling.htm
80kg cyclist, 15kg bike
Rolling resistance Cr 0.005
Air resistance Cw 0.9
Frontal area Af 0.6 m2
Power for 20km/h => 86W
for 40 mins => 465 kCal
This calculation assumes you begin at 20km/h and travel at constant speed on a flat surface with no wind.
Replenishing sugar from body fat
Lipolysis releases fatty acid chains into the blood stream.
In cell mitochondria, a ligase enzyme breaks these down to acyl-CoA. Beta oxidization breaks acyl-CoA and produces acetyl-CoA, which feeds the Citric Acid Cycle. The oxaloacetate from the Citric Acid Cycle is reduced to Malate, transported out to the Cytosol, where it's oxidised back to oxaloacetate then decarboxylated by phosphoenolpyruvate carboxykinase (PEPCK) to phosphoenolpyruvate (PEP). In the Liver, PEP is converted to pyruvate which can be converted (Glugoneogenesis) to Glucose, which is released into the bloodstream. The decarboxylation in the Cytosol is the rate limiting step in fat metabolism.
Assuming you had a normal evening meal and you have breakfast when you arrive, this commute will probably not lower your blood glucose levels to dangerous levels.