If you use a treadmill, don't put too much pressure on yourself at first, just run for 10 minutes, and then increase the slope and speed according to the improvement of physical fitness. But if you want to lose fat, you'd better control your heart rate within the range of burning fat (=(220- age) *60%), and increasing the slope can also cushion your knees.
It does not mean that you must exercise for more than 40min minutes to enter the fat-reducing stage. Doing some strength exercises before running, such as squatting, can accelerate the consumption of fat. And don't listen to some weight-loss advertisements saying that stovepipe or waist and abdomen are all over the body, and only muscle training is local. Therefore, this whole body aerobic exercise has a very good effect on weight loss. After two months, the weight may not change much, but it definitely looks a lot thinner. It is precious water and muscles that lose weight quickly, and we all want to consume fat.
And don't worry about growing muscles. Muscles aren't that long, and aerobic exercise won't make you a King Kong Barbie. I am a girl, too. I have been working out for three months. Not only do I look thinner than ten kilograms, but my physical fitness is much better than before dieting. Now I hope to practice some muscle lines. If I have muscles, I can not only speed up fat metabolism, but also protect us from harm.
I hope my answer can help you.