We mostly post about resistance exercises and calisthenic exercises. Resistance exercises and calisthenics offer plenty of good cardio-pulmonary challenge in themselves, but some almost pure cardio exercise should be part of the fitness mix. The Conditioning or Fitness Triad is what I call it: a total of 2 hrs mostly weights, 1 hr of heavy+lighter calisthenics, 1+ hr of pure cardio.
A few comments about cardio (eg rowing, running, swimming, fast-walk treadmill, stair machine, jump rope, elliptical, bike): These exercises do not build strength. Cardio exercise does nothing to prevent osteoporosis or to strengthen bone. They do help build cardiac power and muscle endurance. These exercises do not help with weight loss in any meaningful way, but keeping your body moving with exertion every day is a good idea, and you can't lift heavy weights every day. We strongly advise against running as a cardio exercise because of joint damage. Sprinting outdoors or indoors is fine, but long runs outdoors are physically destructive (even if mentally beneficial) and offer little fitness benefit.
20-25 minutes of pure cardio, twice or at most 3X/wk, is all anybody needs as the cardio component of a fitness program. It doesn't take much time because it's intensity, not duration, that matters. If you have more time on a given day, spend another 1/2 hr on high-rep weights or calisthenics instead of wasting it on more pure cardio.
There are caveats, however. To be effective, cardio has to be HIIT - High Intensity Interval cardio - approx 1-2 minute sprints alternating with one minute sub-sprints to catch your breath. If you can talk or read during cardio, you are wasting your time. A sprint means around 80% of your max heart rate (unless, like me, you are on BP meds that limit your heart rate in which case you just push it to your max which for me is 120). To get that rate up and maximize the stress, use the max resistance (on a treadmill, for example, an incline of 10 or more) and/or your max sprint speeds and put your heart into it. Otherwise, it's fairly useless unless you are over 80 years old or infirm.
Distance runners (I used to be one) and distance swimmers (I used to swim a mile daily) are not accomplishing very much. It feels good, though.
The other Cardio caveat concerns variety. Continuous repetition of the same cardio exercise increases your efficiency with it and thus diminishes its cardio effectiveness, so we need to vary the cardio exercises to keep the stress up. Mix it up, even within one session. Less boring, too. I always add a few minutes of jump rope at the end - singles, running man, and one-footed, to make sure I finish with nothing left in the tank. Jumping is tough and you will see mostly guys doing it - few if any bouncing gals.
An example of a typical cardio day for me would be 5 minutes elliptical warm-up intervals, set above 10, then a one-minute straight-arm plank, then 15 minutes treadmill fast-walk intervals at a 10% incline, then a minute elbow plank, then 5 minutes jump rope. If I have more free time to fill, I'll do some sets of heavy ball throws, light-weight squats, high-rep curls and press-downs.
