I don't have time to do a cardio and a weight workout. Which one should I give up? – Kathleen Boyd, Stanford, Conn.

There are two myths we can bust here. The first: You have to spend hours a day to get in shape and lose weight. The second: You have to separate your strength training routine from your cardio routine. A type of workout known as circuit training allows you to work your heart and muscles at the same time while burning calories at warp-factor speed.

In a circuit training routine, you jump from one exercise to the next with no or just a few seconds rest in between sets. It's the perfect routine for people who are time-challenged or want to juice up their current exercise routines. Though circuit training won't pack as much of a cardio punch as true aerobic activities like walking and jogging, or as much muscle shaping as slow and steady traditional weight training, doing two to three circuits a week will result in moderate strength gains and good cardiovascular benefits. A 20-minute, high-intensity circuit will burn off upwards of 250 calories, which is comparable to running an eight-minute mile. That's a great overall fitness package.

To complete circuit training properly, include about a dozen strength and toning exercises that hit each of your major muscle groups (legs, buttocks, back, chest, arms, shoulders, lower back and core) at least once per round. Alternate each set of strength moves with one to two minutes of cardio until you complete the entire round of exercises. The idea is to emphasize both hustle and muscle. This is why I recommend alternating upper-body strength moves with a lower- or middle-body moves; it allows you to work your hardest without slowing the pace.

Because circuit training is more intense than your average workout, plan on significantly lowering your usual weights. For example, if you typically use a 10-pound weight to do your biceps curls, use a five- or eight- pound weight for the circuit. And just because you're moving quickly, don't toss technique and safety to the wind: Control each rep, and use impeccable form.

Sample Circuit Training Routine
Alternate the following strength moves with 45-90 seconds of the following cardio intervals: Jumping jacks, jump rope (or jogging in place), high knee running or marching (while lifting knees to opposite elbow). Do 10-15 reps of each strength move, then move immediately into the cardio. Try to take no more than 20 seconds rest in between each transition if you can. The entire circuit should take you between 20 and 30 minutes. If you're up to it and you have the time, rest for two minutes and repeat the circuit.

Order of Strength Moves:

1: Abdominal Crunch
2: Squat
3: Push Up
4: Alternate Lunges
5: Dumbbell Row
6: Kneeling Leg Curl
7: Shoulder Press
8: Back Raise (Super Mans)
9: Biceps Curl
10: Calf Raises
11: Bench Dips
12: Reverse Crunch

My 2 cents: For the flip side of the weight-loss equation, go to How Many Calories Do I Need to find out how many calories you should be eating in order to lose or maintain your weight.

Now for your 2 cents: I'd like to hear from you after you've done this circuit. You can tweet me or leave a comment. For anyone looking for a super-serious circuit workout, be the fifth tweet today, and I'll send you a copy of Personal Training with Jackie, the brand new DVD from TV's hit show "Work Out." I tried it the other day, and trust me, it will hunt down any extra fat on your body and fry it!

Liz Neporent is a diet and fitness expert and co-author of "The Fat-Free Truth." She regularly appears on national TV programs and is the president of Wellness 360, a New-York based wellness provider. You can also follow her on Twitter @lizzyfit.