I Have Just RETIRED .I Am 70 And Very Overweight.

I have just RETIRED .I am 70 and very overweight. I'd like to be able to walk for exercise. I do water exercise 2 times a week,but really think walking would help me. Problem is I have new knee joints. One had an infection and 3 surgeries, so it's been awhile since I've exercised or even walked. I know I need to be active. How do I start to move more?
DianeB