Most people wouldn't be able to run ten kilometers, let alone Normans. She was around 280 pounds. This was like running with weights.
One kilometer was already enough exertion for her. Two kilometers was probably her limit. Chen Zhao's requirement was clearly too much.
Just running didn't actually have a big effect. It mostly exercised the muscles and fat on the thigh. The most effective way to lose weight was actually swimming. It used every muscle in the body.
If she wanted to lose 160 pounds in a month, she had to exercise crazily. Of course, she couldn't do too much in the first two or three days.
Her lazy habits resulted in her body's inertia. Once she started exercising, her muscles would accumulate lactic acid, causing soreness, weakness, and pain. The body could recover by itself though. After becoming accustomed to the exercise, the body would get used to it and adjust to the optimal state.