Your knee hurts after running in the gym for only two days, which means that you are seriously under-exercising, and you are over-exercising.

I suggest you pay 6000 steps every half hour after meals.

Jogging on the gym treadmill for half an hour.

Stand up straight when sitting.

You can do 50+ sit-ups and 20+ push-ups before going to bed.

Pay special attention to what you eat.

Eat more meals a day, but eat less every time.