Sprints. I found a hill in a park nearby, about 60 yards to the top, and I run as fast as I can up it. I walk back down, and after a total of about a minute's rest, I sprint again. I do this 12-15 times, and it takes 15-20 minutes.
Jump rope intervals. I alternate a minute of jumping with 30 seconds rest. I started by doing just standard double-leg jumps, swinging the rope forward, and occasionally doing a set swinging it backward. As I got better, I did more and more backward jumps and fewer forward ones (swinging it backward forces you to jump a little higher to clear the rope and it's much harder). Last week I did all the sets backward and still wasn't tired so I threw in a set of alternating leg jumps at the end (it looks like I'm jogging in place). You can progress quickly with the rope and it burns loads of calories while increasing coordination. As my friend Jon Hinds, owner of the Monkey Bar Gym in Wisconsin, likes to point out, studies have found that 10 minutes of jumping rope burns about as many calories as 30 minutes of jogging. What more do you need to hear?
Walk for 30 minutes. This is where I take it easy with a sort of "active recovery" day. I've actually upped the ante on these lately, adding a few more sessions per week and walking for longer, but the pace is still pretty light. I walk briskly, not casually, around the park. Apart from getting you outside in the fresh air, walking is great because it doesn't put you at risk for muscle loss as other more intense or long-lasting types of cardio do. Muscle loss through cardio, though a little overhyped by fitness pundits these days, is still a big consideration when you're dieting hard. Walking has no downside.
Sprints again. If it's raining, or I hurt myself (I'll explain at a later date), I'll do sprints on an exercise bike. I crank up the resistance and pedal hard and fast for eight to 10 seconds. Rest 45 to 60 seconds (or 90 if I'm really hurting--truth be told I hate these because of the lactic acid burn that builds up), and repeat for 15-20 min.
Off or walk.
Prowler. I already explained how this works last week.
Jump rope for time. This is my only really intense aerobic workout. I jump for as long as I can, rest a minute, and repeat. I've worked up to a set of eight minutes so far, and the whole workout takes up to 20. It's murder on the calves and boring as algebra, but it's a sure fat burner.
If anyone's been paying attention, this is a little different from the schedule I laid out last week. I used to do the sprints on Tuesday and Thursday, but after a few weeks I found I couldn't recover in time for the second session--my legs were sore and the second workout was decidedly slower and weaker than the first. So I adjusted and moved the first sprint to Mondays. The walk and jump rope also flip flopped so I could avoid doing the rope two days in a row and give my knees and ankles somewhat of a break. Being able to make changes to your program on the fly as needed is important. You have to be flexible and balance things without making excuses to take it too easy on yourself.
As you can see, the cardio workouts are mostly pretty short. When they're hard, they should be short. Mentally, it's difficult to do multiple long-duration workouts (like jogging) when you're also lifting three days a week and eating a lot less. I'm not sure that's good for your body either. I know the thinking these days is that cardio workouts need to be long and torturous, preferably until you puke. But as far as I know, there's never been a scientific link found between puke and weight loss (unless we're talking about bulimics, but I won't digress).
Look at sprinters, wrestlers, and boxers. They pioneered the methods I mentioned above and they're some of the leanest people on the planet. This is cardio that works.
Oh, and in case anyone is wondering, I do these cardio workouts fasted, first thing in the morning (with the exception of the sprints). Experts have debated this for years, and I even I have written on this blog about the risks of training on an empty stomach, but I've done it for eight weeks now and haven't had any problems. The idea behind it is that after fasting in your sleep all night your glycogen stores are low and the body is more apt to pull energy from your body fat to fuel activity. The scary alternative is that it could pull it from muscle. This is another reason the workouts are short. As soon as I get home I drink a shake with protein and carbs, but I don't feel I've lost any muscle yet.