When I put people on cardio equipment (bikes, cross trainers, ellipticals, steppers, rowers) I tell them the heart rate I want them to achieve. And they look back at me as though I'm mad because they've seen a chart like the one here (in fact, it might even be on the piece of equipment they're standing on).
I may also mention I want them to ignore the bottom two zones entirely - well, less mention more cover it up with my hands and tell them to not even go there!
See, the point is, exercise is there to get you fitter, not necessarily to help you lose weight (the weight loss is secondary), and to get fit you have to push your heart rate above your comfort level - at least to the 70% of your max HR example I used in my last post. Even then, once you've been working out for a little while, 70% will seem too easy and you can push yourself further. The higher you push yourself during the workout (within reason: no fainting or vomiting please) the longer your metabolic rate stays elevated afterwards, which means your body is burning calories for longer than if you stayed in the Fat Burning Zone of under 70%.
The fat burning zone came into being because someone once worked out that the percentage of calories burned from fat was higher when your heart rate was lower. To burn fat, your body need oxygen, so the hypothesis was that if you push yourself too hard and start gasping for breath, the fat burning percentage reduces, this isn't entirely untrue...
BUT, it completely ignores the fact that when you work out harder, you burn more total calories...
If I was on the cross trainer for 20 minutes working out at 60% of my maximum heart rate, I would burn say 150 calories, half of which come from fat, so 75 calories from fat.
If I was on the same cross trainer working out at 80% of my max, I could burn 250 calories, but only 35% from fat. But that equates to roughly 87 calories!
Please bear in mind these are not accurate figures, they vary from person to person, but they're a good guide to explain why you should be pushing yourself a little harder.
MY SUPER EASY INTENSITY GUIDE
65% or lower - you're able to talk non-stop about last night's TV, all of it from 7pm to 10pm - TOO EASY
70 - 80% - random sentences, taking a pause in between to catch your breath - CARDIO/BEGINNER
80 - 90% - gasping, unable to talk, but feel great and happy to continue - INTERVALS/ADVANCED
90 - 100% - almost maximum effort - INTERVALS/NOT FOR BEGINNERS
A mixture of heart rates between 70 to 90% is probably the best bet for most people - remembering that if you feel dizzy or faint, you're pushing yourself too hard and need to slow down/lower the intensity level on the machine your using - and the fitter you are the easier it will be to raise and maintain a higher heart rate.
This post has been weight-loss centric, but your fitness will also benefit from working out at a higher intensity - the higher you can push your heart rate for longer, the more your endurance will develop. Once you have a good base of endurance fitness, you'll be able to breeze through every new fitness class your gym offers and every run your partner/friend drags you out on.