Okay, so what I really want to do is burn up my fat, because what's the point in being light if you have a high body fat percent?
So I've been doing some reading, and while a lot of magazines proclaim that walking at a low intensity is best for fat burning, it's not true. The main reason why magazines and people recommend walking is because it's easier for an unfit or normal person to walk than run, energy levels while on a diet are generally too low to allow for strenuous activity, and people don't really want to be told that they have to do a lot of hard work to get results.
And since some of those are true, namely that I doubt many of us could keep up a run or heavy jog for that long, unless we've been trained, interval training is what experts recommend. Basically, running at a pace you can manage for two to five minutes, then walking for two minutes, then running again, for as long as you are capable, preferrably 15 to 30 minutes.
To know if your body is in optimal fat burning mode, you need to figure out your heart rate. It's really easy to do, just take your pulse in your wrist or neck with your index and middle fingers [not your thumb, since it has its own pulse] for ten seconds, then multiply that number by 6. For example, sitting here, my resting heart rate is 13 beats in 10 seconds or 78 beats per minute [bpm]. The average healthy resting heartbeat for men is 70bpm and 75bpm for women. It ranges from 60 to 80bpm for healthy individuals, and rises with age, and is generally lower in healthy, fit people.
Anyway, your optimal heart rate for burning fat is between 65% and 85% of your maximum heart rate. To find out your maximum heart rate, subtract your age in years from 220. In order to find that range, multiply the result by .65 and .85 respectively. For example, I am 17, and so my maximum heart rate is 203, and my range is 132 to 172bpm.
To make sure your heart rate remains in this range, just take your pulse as mentioned above every ten minutes while exercising.
The advantages of high intensity exercise over low intensity are that it raises your metabolism higher, and for longer afterwards. It also builds muscle, which is good because that raises your resting metabolism. And last but not least, while high-intensity exercise only burns 40% of calories from fat, and low intensity burns 50% of calories from fat, the amount of calories burnt will be higher in high-intensity exercise, unless you're doing a lot of low-intensity exercise. The numbers I got from one site are that 20 minutes of walking burns 100cals, while 10 minutes of high intensity interval training burns 160cals. From the two kinds of exercise, you are burning 50 and 64 calories of fat respectively.