We are a commune of inquiring, skeptical, politically centrist, capitalist, anglophile, traditionalist New England Yankee humans, humanoids, and animals with many interests beyond and above politics. Each of us has had a high-school education (or GED), but all had ADD so didn't pay attention very well, especially the dogs. Each one of us does "try my best to be just like I am," and none of us enjoys working for others, including for Maggie, from whom we receive neither a nickel nor a dime. Freedom from nags, cranks, government, do-gooders, control-freaks and idiots is all that we ask for.
There are many things called that people term "cardio." However, Cardio means highly-elevated heart rate. Nothing wrong with ordinary swimming, jogging, stair-climbing, walking, rowing, etc for recovery days, but they do not do much for conditioning except for the infirm.
For cardio conditioning, use brief all-out sprints in all of the above. 30-60-second sprints, then a 60-second breather before the next one. That's HIIT - a good component of fitness training.
It took me a while to go from jogging to sprinting, but it has been worth it. Sprinting is an endurance-builder.
Still, as we say, doing anything is better than sitting.
I'm an ex oarsman who is now doing Crossfit. I'm definitely stronger. However despite lots of HIT, and shorter high intensity workouts my endurance has deteriorated noticeably. (I'm getting older, but of course that can't be the explanation...) A long interval workout may be 20 min. More normally they are 8 - 12 min.
What's missing is the long base building that I used to get rowing a couple of years ago; 45 min - 1 hour of steady state work with a heart rate somewhere around 160 bpm.
Depends on what kinds of exercise you do. There's actually a lot of science behind how to build up fitness in particular aspects of your endurance, based on heart rate or power output zones. Here is a good place to start.
I raced bikes with some former rowers. They were extremely tough in high intensity 40-60 minute road and cyclocross races. They had to work to build up more sustainable endurance that a lot of traditional bike racers - used to three hour roadraces - had. And the longer distance guys struggled in 45 minute criteriums, which tended to be 20 -40 lap four corner sprints.
The science of zone training is pretty advanced, BTW; the various Warsaw Pact Institutes for Sport Science (Olympic training programs) pioneered it (along with better recovery methods, how to dope effectively, selective breeding programs, and so forth... didn't say they were ethical, only said they were effective).