Let me begin by saying I've never had children or a serious knee injury (with surgery) to understand fully what you've been through. I've also never been married, but I know relationships and the pressures that come with it - especially for women and their image.
One thing I do know is what has worked for ME: exercising and eating for the right reasons... with surprising results.
My best advice is to alter your perspective about becoming healthier. Don't think about how you "need to lose weight" or even "want to lose weight" but rather, "I want to be healthy." This includes having a healthy heart, mind and more. You can train yourself to eat healthier so that you can live longer for your child and husband. You can also start by exercising as a get-away (walking for example, like stated above) and eventually you'll increase your stride into running. You don't need a gym to do this. Just some fresh air and a sidewalk or trail.
Let me provide an example. I used to think peppers were gross. But then I read they gave you energy. Knowing I wanted to have a great mood throughout my workday, I started telling myself these peppers were tasty and are one step in the right direction. So I started adding them to meals any possible way I could without overdoing it.
In terms of exercising... Running completely exhausted me. I always got shin splits, it was too cold outside, or I put so much pressure on myself to run 3 miles a day. What? Such a random (and extreme) goal to set for myself. I tossed that quick. I started yoga and found such joy in it. Then I started going to outdoor basketball courts in the spring/summer to shoot hoops. I had always played basketball growing up, so this was a great way to just move a little more than I would have being on the couch. Maybe this could be something you do with your son? My point is that you should start small... until it becomes a lifestyle.
I promise you though, if you choose to do this, you must do it for YOU. Yes, you want to be more attractive to your husband and healthier AND happier for your child (and possibly future children), but this is your life. You are most important. It's not selfish either.