One of the most significant exercise studies done in the last half century was a 1975 West Point Study. You can read the details in the link, but in a nutshell, West Point wanted to, among other things, “identify the consequences of a short duration, high intensity strength training program.” To that end, cadets were divided into groups a control group, that continued traditional training methods consisting of hours in the gym and on the track, and a strength training only group (referred to as the “whole body group”) which trained 3 times a week for short durations using only Nautilus strength training machines. The whole body group was strictly supervised, and they participated in no running or other training. A battery of strength and cardiovascular measurements were taken at the beginning of the study (2 weeks in, to allow for any false improvements.)
At the end of the 8 week study, naturally the whole body group had greater improvements in terms of strength. But what was surprising was that “On NONE of the 60 indices purporting to evaluate the effects of the training on the cardiovascular function was the control group better on the final testing period (or on the change from initial to final) than the whole body group.” The whole body group even had more improvement in the 2 mile run, and they had not practiced running in 8 weeks.
Col. James Peterson, who conducted the test concluded, “Contrary to most commonly held beliefs on the subject of strength training, the training also significantly improved the cardiovascular condition of the subjects. By maintaining the intensity of the workouts at a high level and by limiting the amount of rest between exercises, the training resulted n improvement on each of 60 separate measures of cardiovascular fitness. Contrary to widespread opinion, not only will a properly conducted program of strength training produce increases in muscular strength but will also significantly improve an individual’s level of cardiovascular condition. The data suggests that some of these cardiovascular benefits apparently cannot be achieved by any other type of training. And finally, the experimental subjects increased their level of flexibility by an average of more than 10% on the three evaluative items.” (Emphasis added).
This study, and/or aspects of it, has been repeated throughout the last 40 years, with similar results.
So yeah. I “think” 30 minutes of high intensity training can replace hours and hours of “cardio.”