20 minutes of exercise is a very reasonable time to start...
If you want to burn lots of calories try doing HIIT -- High Intensity Interval Training... It's quick but not easy
... Here is a great
site explaining how it works.... It can be done with biking, running, walking any type of exercise really....
Good Luck!!
P.S.: Just make sure that your eating plan is very clean, by this I mean that it should be as unprocessed as possible.... The food in your diet is 80% of the equation to successful weightloss....