I agree with the doctor and would also like to add, that you should make sure you choose exercises that you enjoy, because you won't stick with it, if it's not fun.
Hi!
Well, that’s a good question. There is no golden rule to how long you should exercise. For most people a 30 minutes exercise 4-5 times a week is sufficient. However, no two individuals are same. The goal is to stay into exercise for life. So begin at a pace and duration you are comfortable with. Something that does not tire you! It may just be 5 mins. Though today it will not cause any wonders, with time you will stick to it, slowly increase your pace and time and get wonderful results. Take care!