How exercise really works is poorly understood by most people. Let me give you myself as an example. I'm about 5'10" and weigh about 230 lbs. Even though my BMI says that I am borderline obese, I actually have about 20% body fat (not too bad for someone in their 50s). I am very muscular and very strong for someone my age. This probably doesn't seem very relevant to you, because you don't have the time to devote yourself to a rigorous exercise program, and besides you probably aren't very good at athletic stuff either.
The entirety of my rigorous exercise program: I lift weights for 15 minutes or less, once a week (and often only every other week).
I always hesitate a little bit to tell people this, because I realize it sounds absurd. It can't possible be true. If it is true, it must be because I'm just genetically gifted and naturally muscular. Or I must be taking steroids or human growth hormone. The reality is that I am a scrawny math and computer geek, not particularly athletic at all. I weighed 155 lbs. when I was 20 years old, and I had so little muscle then that I was still overweight. I could barely complete three sets of 10 on a bench press with an empty bar. I kept at it though, and over the course of several years put on lots of muscle. I did this by working out for several hours a day, six days a week. But once I got out in the real world and had a job, it became harder and harder to find the time to work out. By the time I was 30, I had not been lifting weights much for a few years, and was not as lean as I used to be.
Then I tried a form of weightlifting called SuperSlow (and have been doing it ever since). The idea behind SuperSlow is that you only do five or six exercises, once a week, taking 20 minutes for your entire workout at most. The trick is that you do each exercise very slowly (15-25 seconds per rep), going even slower during the hardest part of the lift. You only do each exercise once (i.e., one set only), but you don't worry about the number of reps you do. Instead, you keep going until you just can't do another rep. This is called "going to failure" and is the hidden key to building muscle effectively. Once you finish one exercise, you get to the next one as quick as you can (30 seconds or less, ideally). You want to minimize the amount of rest you get and maximize the intensity of your workout. Not the length of workout, not the amount of weight you can lift, not the frequency. Just the intensity.
The reason this works is because building muscle doesn't work like other forms of exercise. The only time your body builds muscle is when you are in deep REM sleep, and even then only when your body has released growth hormone to tell your body to add muscle. Your body does not want to add muscle. Muscle takes a ton of calories to build and then lots more just to keep it alive. Your body is evolved to conserve and store calories as much as it possibly can (which is why it is so easy to add on fat). Your body thinks there is always a famine just waiting to happen and it can't use precious calories on something as frivolous as extra muscle. You have to convince your body that if it doesn't build more muscle, you are going to die. This is done most effectively by putting your muscles under a moderately heavy load, lifting that load for about a minute and a half to two minutes, then continuing to try to lift that load for up to ten seconds after your muscles have weakened so much that you can't actually move the weight anymore. This failure to move a heavy load, that you were able to move just moments ago, is the primary signal that tells your body to release growth hormone and build muscle when you sleep. This is how you convince your body that famine is not the danger--being too weak is the danger and you must be stronger.
If you do SuperSlow correctly, the signal your body gets is so strong it can persist for about a week. If you lift weights more often than that (even though your body will be able to do it), you are just forcing your body to recover from your workout instead of build muscle. It already got the message, you don't need to keep repeating it. You just need to give your body the time to do what you told it to do. The most common mistake weightlifters make is over-training.
There are lots of limitations that come from doing SuperSlow. First, you need to use machines, not free weights. You are going to failure and doing this with free weights is just too dangerous. You also need the machines you use to be high-quality and well-maintained so that they still move smoothly when you are going really slow. Most normal gyms don't have machines of this quality or don't maintain them well enough. Second, trying to go to failure is really hard. Your body and your mind will fight you and try to get you to stop before you've really gotten to failure. To be really effective, you need a trainer (who is certified in SuperSlow) to help you past the physical and psychological barriers, as well as to help you perfect your form on each exercise (which is important for both safety and effectiveness). Third, all of this costs money--generally $45-50 per session. You can do limited forms of SuperSlow without a trainer and without equipment, but it won't be as effective (but will still be much better than what most people do).
So, if SuperSlow is so effective, why don't professional athletes use it? The problem is that SuperSlow is super-effective at building muscle, not at maximizing athletic performance. If you want to run marathons, you need to run a huge amount, not put on muscle. If you want to be better at a particular sport, you need to practice the skills of that sport, increase your endurance, your jumping ability, your speed, etc., not just build muscle. Even if you are an elite power-lifter, you usually don't need more muscle, you need to train the muscle you have to fire as fast as possible (increasing your power). If you maximize the amount of muscle you have, you'll just move up to a higher weight category and end up competing with lifters who have bigger frames. If your primary concern is athletic performance, SuperSlow is not for you (you don't want exercise--you want athletic training).
Isn't doing cardio work more important than building muscle? Cardio work is important. Fortunately, a SuperSlow workout is intense cardio, far more intense than most aerobics, treadmills, cycles, etc. A good SuperSlow workout is like sprinting for two miles. I lay on the floor for five minutes after I finish a workout just to recover enough to stand (and it was fifteen minutes when I started and had not developed my heart and lungs enough). Make sure you don't mistake endurance work for cardio work. If you want to be able to run long distances, do triathlons, etc., then you need to do endurance work (miles of running, swimming, cycling, etc.). But if you just want your heart and lungs to be stronger, SuperSlow works great for that (endurance exercise works well too, but it takes a lot more time and effort).
But I want to lose weight, so that means I need to run, swim, cycle, etc., right? No, not at all. Lots of endurance exercise will cause you to lose weight, but that's doing it the hard way (for most people). Muscle burns calories 24 hours a day, even when you are sleeping. That extra 50 lbs. of muscle I have? That's an extra 750 calories a day I burn just sitting in my office and sleeping. That's equivalent to the entire calorie intake of an average meal. If you build muscle and become stronger, you will lose fat as a side effect, even if your weight goes up. Don't worry about how much you weigh, worry about how much fat you have. Putting on muscle is by far the easiest and most effective way to lose fat (unless you are a natural endurance athlete).
I have high cholesterol, high blood pressure, etc., so certainly I have to get more endurance exercise, right? Nope. You desperately need more muscle. It is highly likely that you are on the path to diabetes because your fat cells are insulin sensitive and your muscle cells are insulin resistant. This means your body is packing any calories it can in to your fat cells. You need to reverse this, making your fat cells insulin resistant and your muscle cells insulin sensitive, which you can do best by adding muscle. Diet matters a lot here as well (see Nutrition Basics), and for some people medicine will be necessary, but adding muscle helps a bunch. If you really enjoy endurance exercise, then it can certainly help too, but for most people, adding muscle is much easier.
It sounds dangerous. Won't I get injured? Nothing is completely safe, but SuperSlow is the safest form of weightlifting there is, and is much safer than running, jumping, cycling, swimming, etc. Most injuries are caused by a sudden acceleration at just the wrong time in just the wrong way. Force equals mass times acceleration, and the force is the problem. While there is a decent amount of mass involved with SuperSlow, the accelerations are incredibly small. SuperSlow involves keeping the weight under perfect control with absolutely minimal acceleration at all times. When I start a leg press, I'm applying force for 20 seconds before the weight even starts moving. SuperSlow was even originally invented for use by little old ladies with osteoporosis as a safe way for non-athletic people to build muscle.
I'm female. What if I don't want to look like a female body builder? Don't worry. Unless you have very unusual genetics and are taking lots of steroids, you will never look like a female body builder. Even if you feel like you are getting a little more muscular than you would like, you can just decrease the frequency of your workouts so you don't add too much muscle. Or you could just realize that a muscular, fit, strong woman is awesome and so much better than the anorexic, unhealthy, weakling that the fashion magazines want you to be. Your choice...
What if I can't afford a trainer? Try doing as much as you can at a regular gym or just do push ups and pull ups. Go as slow as you can and go to complete failure. Don't rest between exercises. Focus on the big multi-joint exercises (leg presses, chest presses, overhead presses, rows, pull-downs, etc.). Don't do single-joint exercises (most curls, extensions, calf raises, etc.), which are generally just a waste of time and energy (they aren't intense enough to give your body a strong signal). This won't be as good as real SuperSlow, but it will likely be better than what you were doing before. You might have to do it twice a week if you can't get the intensity high enough, but be very careful about over-training. Also, consult your doctor if you have any worries about medical conditions that might be a problem.
It still sounds like magic. Is there any scientific evidence behind this? SuperSlow was invented and perfected by a combination of doctors and elite trainers. It is deeply grounded in science and has been studied extensively. If you want to know more about the actual biological mechanisms at work and the science behind all this, read Body by Science, by Doug McGuff. This is the best book ever written about exercise and fitness (not athletic training, though, which is completely different). But honestly, it's not any research that convinced me SuperSlow works--the results that I personally have gotten are what convinced me. The results are not subtle or hard to notice.
Okay, you've convinced me to give it a try. What now? Find a SuperSlow trainer near you. You can just do an internet search, but if you are in the Seattle area, I highly, highly recommend the SuperSlow Zone in Capitol Hill. Brian, the owner, is the best trainer I've ever had (and I've got a degree in Kinesiology, specializing in weight training, from one of the top schools in the world for the subject). Tell him I sent you (the first workout is always free).