Years ago, back when I was young and still in the army, a friend and I used to do circuit training every day. It was called just that, back then: circuit training.
The idea has been revamped now, as these things tend to, and it's now referred to as High Impact Training: HIT for short.
The concept is simple. You work hard (bloody hard) for a few minutes, then you have a brief rest period whilst your heart and breathing recover slightly (but not fully). Then you do it all again. Apparently (and I remember very clearly thinking exactly the same all those years back), the results of a 20-30 minute work out two or three times a week are more beneficial than a regular 1 hour steady run. As with everything, you have to balance these things though (I will need endurance training, as well as HIT).
But the thing with HIT is, it hurts. It hurts A LOT.
I'm talking on your knees, shaking, gulping in huge lungfuls of air, dripping sweat onto the mat as your heart thunders against your chest kind of hurting. If you incorporate six or so exercises into a set, you are looking at doing four to five sets in half an hour. You push yourself, and you keep pushing. And when you rest you think, 'In a few minutes, I've got to do all of that again.'
An hour later, and the adrenalin is still causing
One time soldier, part-time author, full-time training manager, husband and father.