Canadian Sports physiologist Trent stringwhorf explained that the so-called fast running means that the last meal in the morning is eaten before dinner the day before, and only drink some water or coffee before running after getting up in the morning. The goal of fast running is to get fat in the cells into the bloodstream, but the process takes 45-60 minutes.
If you can only run for 20 minutes, theres no need to try fast running. Its not worth it unless you can guarantee at least an hour of running every morning. Said stirringworth.
Fasting running is similar to low glycogen running. When running, the glycogen in the body is at a low level. Running will almost exhaust the glycogen stored. However, low glycogen does not mean low energy. Although the carbohydrate intake of runners is almost exhausted, they need to supplement fat and protein to maintain their energy needs.
However, stringwov points out that fasting running may have more side effects on women than men. Male runners can maintain longer exercise time with lower energy before bone mineral density or hormone changes, but female body cant maintain longer exercise with fasting