1. At least 20 minutes a day depends on physical endurance. If you really can't hold on, rest.

2. Lie on the bed and lift your legs like riding a bike.

The result is a certain degree of stovepipe, but long-term exercise like this is not good for the waist.

4. It will really have some effect.

Finally, my own suggestion. There are many ways to stovepipe. It is not recommended to do so much. The main stress is your back. Doing this exercise is also a time to test your waist strength (it will damage your waist muscles). I suggest you really ride a bike. If possible, go to the gym and use the treadmill to slow down. You should walk for at least 40 minutes. Sweating in the first 20 minutes, fat flow in the last 20 minutes. The most effective way to lose weight is to spin in the gym.