Focus on Cardio. What do I do?

Guys, I just set up my Zwift stuff. I've never been a cyclist, but I used to run 4-8 miles a day. Of course, that stopped about 10 years ago. Now I'm fat, 40, and out of shape. My focus at this point is on building my cardio. I smoked for 20 years and have been super idle for a decade. Most of my strength seems to have remained, but due to the cardio problems, I can barely make use of it. I must stress that however bad you think my cardio is, it is worse. Way worse. I get winded walking my dog. I might even get winded watching someone else exercise.

So my question is this. What do I need to do to build my cardio back up? Timewise, I have time to ride for as long as needed pretty much every day. I'm inexperienced, so a lot of the concepts and such are over my head at this point. I know I can just free ride for an hour a day, but I was hoping for something a little more structured.