Earlier quoted context omitted.
It only takes a few months to make substantial gains in c-v fitness.
Define substantial. I find that incredible. EDIT: Now that I've seen the replies, I realize that this is from a very low baseline. I'm personally going to the gym four to five days a week already and my own cardiovascular fitness has not seen much of an improvement in many months.
If you've never lifted before and you hit the gym consistently and have the proper diet, you will get some serious "newb gains." The first year can be amazing.
But if you've lifted seriously for a decade, it's totally different. Muscle mass is very hard to add now.
Cardio gains from a coach potato introducing a serious routine will be incredible. I went from not being able to run more than 30 seconds to running a 5k in 4-5 months, and that's without pushing myself extremely hard.