Intervals have been around forever. The problem is that out of shape guys go directly to intervals and they do the work period for such a short period of time that it is isn't really effective. Try something like after a warm up 200 yard sprints. Maybe 8 x 200 meters with a a 200 meter walk/slow jog. Most want to do 40 yard sprint intervals when they are fat and out of shape. It a stress to the heart and primes you for an orthopedic injury. Intervals are really dangerous if you have bad heart because of the redline immediate stress.
Doing conventional mainly aerobic type conditioning is also a really good method but the bro science and one week of internet schooling personal trainers have given it a thumbs down. The reason I suspect is because it's tough to go out and run a couple miles. It's to much work for the lazy who wants to run six 40 yard sprints and go home.
