Question: how many calories 20 km?
Answer: I think you may mean a 60kg person walking at medium speed for 20km. You will 'burn' approximately 300 calories (circa 1,300 kJ) an hour at say 3 to 4 mph (5 to 6.5 kph). So at a nice moderate stroll (3mph/5kph) you'll take four hours = circa 1,000 to 1,200 calories/5,000kJ.