Cardio, especially that of the long and sustained variation, gets a bit of a bad rap these days, but in my opinion, it does have a time and place. It comes back to training for an objective.
1. If you're training for an endurance race, there's only so much mileage you can get out of training for it using shorter interval work. Sorry, but unless you're a beginner or you're protecting injuries from high volume training, you need to train volume to "perform" well at volume. Notice I said "perform". Performing isn't implying that the objective is to merely cross the finish line, it's implying the objective is to be competitive.
2. If you're trying to lose weight or size. You should note that there is a distinction between weight loss and bodyfat loss. They are, for the most part, not one and the same. If you're trying to build or maintain lean muscle, running on a treadmill for hours is probably not the ideal solution. As a general rule weight training has the power to change the shape of your body, while long steady state cardio is more likely to make you a smaller, and potentially "skinny fat" version of yourself. In my experience, a combination of long sustained cardio and high intensity intervals is the most effective for fat loss. In my 20's running served me best. In my 30's, I find power walking just as, if not more, effective.
The argument for fasted cardio: If training for fat-loss, fasted cardio can be beneficial as performance is not a factor. It puts the body in a lower-insulin state during exercise, which will suppress ALPHA receptor activity (fat storing receptors). It also causes us to increase catecholamine hormone production (adrenaline/noradrenaline), which now can preferentially increase BETA receptor activity (fat releasing receptors). However, if you need to “perform” ie you are training for a race or other specific event, training without adequate fuel could be detrimental to you hitting your required numbers.
3. As recovery. I often program a long run, row or ride after a "hard" day or two. If nothing else, it gets the circulation going which in turn helps with reducing delayed onset muscle soreness. However there is always the danger that ego gets in the way and if you feel good, you go harder. Don't. Go hard on your hard days and easy on your easy days. Avoid the grey area of junk miles in between. And choose your recovery wisely. If you're a decent runner, then running can act as recovery for you. If it's not easy for you, don't be a hero. Pick something else that is, otherwise you're completely missing the point.
4. To build base fitness. The bigger a foundation you have when it comes to cardiovascular fitness and strength endurance, the more you can build on top of it. I always recommend 6 weeks of aerobic exercise to my newer and less experienced clients. The fitter you are overall, the better you can hit the intensities required when it comes to training in high intensity intervals. Too often people don't understand that to get the true benefits (and elicit a specific hormonal response) of interval training you have to be able to go relatively hard in the first place. And we should clarify that if you're doing a true HIIT (high intensity interval training) set, you should be able to handle no more than 20-30 minutes. HIIT is not jogging for a minute then resting for a minute.
5. For building and maintaining a well rounded continuum of energy systems to make sure you can do anything you want any time you want! Some of you will have specific objectives, whether they be aesthetic or performance related. Some of you won't. Some of you will have to sacrifice parts of the fitness spectrum in order to make gains in other parts. The trick is to know your objective and train for that. And being a jack of all trades is a valid objective.
6. Next time you're in the weights room, put together a circuit of 5-6 full body exercises, and complete them consecutively with minimal rest. Did you experience a cardiovascular challenge? Given that the definition of cardio is to "raise the heart rate", what does this mean for for weight training? Are cardio and weights mutually exclusive or merely same same but different? Food for thought right! #justsaying