Generally, it is required to maintain the heart rate at 120- 160/ min for 30 to 60 minutes. Because aerobic exercise initially consumes glycogen in the body to provide energy for muscles, it takes about 20 minutes to half an hour to consume glycogen stored in the body, and then it consumes energy provided by fat, so aerobic exercise needs to last for a certain period of time.
If you run too slowly, you will consume less energy and glycogen, so you will consume less fat in the same time; If you run too fast, the muscles will undergo anaerobic metabolism and lactic acid will accumulate in the muscles. When accumulated to a certain extent, muscles will be sore and energy will not be supplied. If you run down, your muscles will spasm, so you basically won't consume fat.
So running needs to choose a suitable speed. The most suitable speed is the fastest running speed under aerobic metabolism, which needs to be adjusted by yourself. This should make the muscles feel no pain after running.