If you are obese and sedentary

Say you haven't swam in decades And you work a desk job sitting down all day and then you go home and sit down all day as well playing video games and you've been living this way your entire adult life

How much would swimming 3-4 times per week (30-60 mins per session 1+ kilometers ) improve your health/weight/body and how many weeks would it take to start noticing results?