Disclaimer: I'm not lecturing anyone here. As usual, my hatred of the complication of fitness as passed off by commercial businesses has led to a longer post than I intended...
The important thing is to challenge yourself. Walk for 30 seconds, jog for 30 seconds, and run your maximum for 30 seconds. See how many of those you can last, and give it 200%. When your legs rest up a couple days later, do it again. The objective is to work on your lungs.
There is so much idiocy in the fitness world that it's staggering. Here's a factoid: sit-ups are utterly, completely worthless unless part of a circuit training routine, and they're only good for cardio as part of that. We all have six or eight packs. But most of us have them obscured by a layer of fat. Sit-ups will do nothing but increase the overall endurance of your abs. They won't melt away the fat, and neither will they develop the muscles so that they somehow show through. Working a part of your body does NOT remove the fat there. When you work out, your body removes fat from all over in a balanced way. That's why you don't see utter blobs with perfect, washboard abs, or overflowing pear-people with perfect, chiseled legs from walking.
Health begins with your heart and lungs, and challenging yourself with hard sprints is the quickest way to health, even if you're well over your acceptable body weight. If you have developed cardiovascular health, you can live jollily overweight and still enjoy great performance (though not your maximum, by any means). Even in that scenario, you can eliminate risk of heart disease, some forms of cancer, and other causes of death almost as much as your thinner companions who are also in good health. Studies have shown that those who sprinted even for 30 seconds as part of a walking workout of a few minutes saw greater gains in well-being than their slower peers.
Weight loss begins with long work. If you want to lose weight, it's perfectly fine for you to walk 50 miles a day. But if you do not challenge yourself (being out of breath means it's working), your cardiovascular health won't improve much. To achieve both, develop your wind until you can jog freely. At that point, you can work off weight a lot faster than those who must still walk because their bodies simply can't handle going at a faster pace. The longer you work, the more cutaneous fat is burned.
All you need are a track and two feet (and a physical to make sure you don't have some kind of heart condition before working at your maximum). Nutrisystem, elaborate plans, wonder-machines, and paid trainers are all compensatory tools for a lack of will power. If the few articles I've read lately are right, at some point in the diet world the self-will was written off as an inadequate tool and restrictive, slavish plans were imported to keep one's habits in check. To those who wish to improve only as a passing desire, these mindless methods may work. But to those capable of visualizing success, seeking it logically, and unleashing their passionate will to improve themselves, self-will and some good cross-training shoes are all one needs. I pity people who pay small fortunes for diet plans and special treatment. Protein shakes are worthless; unless you live in the third world, chances are you consume enough daily! Gatorade and other energy drinks corrode your teeth and are meant for fast, transitory performance during athletic games, not overall health and endurance. And the other steroids are all not worth the health risks involved. Some of the fault lies with the companies who appeal to their ignorance with "easy plans" or proliferate misinformation about physiology. Science alone prevails!
