Thomas, my two cents:
No, your metabolism is not slowing down. Your activity level just isn't nearly what it used to be. So, you should not be surprised that you're heavier than you used to be.
You can do cardio. But it is not necessary to lose weight. What you really need to do is dial back the calories in a manner which is sustainable for you. Fair warning: you can't out-exercise overeating.
For exercise, my prescription:
Dose: one set to failure.
Frequency: once or two times a week at most.
Then get on with your busy life and watch your body change. You're probably 'shirt big' right now. Lose the fat and build some muscle and you'll eventually look great without a shirt.