The best way to burn fat is to get your heart working at 75% of your maximum heart rate for at least 15 mins.
Your heart rate is calculated at 220 minus your age.
You'll burn as many calories walking a mile at a good pace as you will by running it. So if you run out of puff jogging, walk. Or jog and walk, jog and walk, and so on.
Aim to do your 30 mins exercise but also shave 300 to 500 calories off your daily diet, so you target weight loss on two fronts.
Basically, if you can have a slightly breathless chat with someone as you exercise, you're just about on the button for fat burning.
Going to hard at exercise means that you're simply strengthening your cardio-vascular system, which explains why many women who to to aerobics don't actually lose weight, but feel much better!
Burn the weight first, then go to the cardio-vascular stage.
Oh - and forget the scales for now.
Buy a belt, put it on and mark it with a pen.
Once a fortnight, put the belt on and mark it again. Much more effective than losing sleep over a pound or two here and there.
I used to be 17st 3lbs. (I'm 6ft 2ins tall).
I'm now 14 st. It worked for me and took a year.