In the general fitness and endurance category of exercises (as opposed to strength and power), bursts of "balls to the wall"* (not sure what term women might use for that) exertion are far more useful than long stretches of less intense cardio. Except for those over 75 or in cardiac rehab, long stretches of "comfortable cardio" advance none of your goals other than plain functional maintenance. Functional maintenance is good, though, for the aging. (Comfortable cardio means you sweat but do not get winded.)
You can watch Social X-Rays, models, and quasi-anorectics jogging or doing cardio exercises for hours all the time in NYC, but they do not look like that because of exercise. They look like that because they don't eat carbs, or anything else much. Creepy.
Does high intensity interval training work?
What most of us regular exercisers do is a combination, eg 20-40 minutes of continuous cardio machines with brief bursts of maximum speed effort - sprints in effect. Distance runners train with sprints, as do swimmers. Other high-intensity sprint-like exercises? Squats, heavy ball smashes, lunges, high steps with weights, burpees, and the like. Basically, any relatively-brief spurts of demand that make you feel like you might drop dead, or might wish to drop dead.
Why short bursts of activity boost fitness in the body. You make more mitochondria, for a while
Remember, cardio doesn't burn fat hardly at all, and does not build strength or power either. It can burn some carbs (roughly 2 slices of toast per hour), help mobilize fat-burning somewhat only when combined with low carb, and it puts whatever strength and power you have to work. It is really just about endurance/fitness. If you build your strength and power with other exercises, your effectiveness with high-intensity will obviously increase because your maximum will be more intense.
Losing fat is low-carb diet, improving fitness is demanding cardio, and improving strength/power is resistance. It's simple. Our goal is simple too: to be fit for whatever life offers.
*Thanks to reader for etymology of the balls expression
