Biking!

Pepper, I'm sorry but there are a few statements that are physiologically just plain wrong and they can not be left like this.

"exercise itself usually doesn't burn fat "

Exercise itself DOES burn fat!

'Fat is used aerobically and for most of us it is solely a low-intensity fuel. It supplies most of the bodies energy needs at rest and even during exercise up to a medium intensity. ... We store 50,000 to 200,000 kilo calories as fat. Completely oxidized fat yields 9 kcal/g or 38 J/g. Enough energy for 100-200 hrs of hard work (or more realistically, 200-400 hours of moderate work with rest. Stored dlucose and glycogen can furnish only 1-2 percent of that amount of energy.'

'Experts imply in their papers that low-power carbohydrate oxidation ( as occurs in cycling during easy pedaling) facilitates fat oxidation, whereas high-power oxidation actually inhibits fat use'

For more details see D. G. Wilson, Bicycle science 3rd edition, 2004 , or Holloszy, J.O. and ED.F. Coyle (1984) "Adaptations of skeletal muscles to endurance exercise and their metabolic consequences". Journal of Applied Physiology 56, no4, 831-838

Lots more to be found if you Google it. Or research databases.

Now that is not MY opinion, this is based on extensive and intensive research. It also is as far as I know the theory that experts agree on. I am not aware that it might generally be disputed. I could be wrong.

"So, unless you burn more than 600 calories (and probably more than that, if you eat before you exercise), you do not burn fat when you're running, biking, etc."

That is true, and that is the reason why you have to exercise for a while (in your aerobic zone) before your body starts using fat as an energy source. However after that, fat is a major energy source. Not the only one, but the primarily used one.

"Granted, they also don't have any fat to burn"
Every body has a reserve of fat. MUST have. It is what the body uses to create energy for long term steady efforts. If we had only the glycogen stored in muscles and liver any physical effort would be very limited. Again, it depends on whether your efforts are causing aerobic or anaerobic processes to be used.

"It's those times when you feel like you are running on fumes when you are most likely to be burning fat"

I read "running on fumes as being way up in your anaerobic zone. Meaning your body is producing energy now without using oxygen. Fat can not be used anaerobically, it is a slow, oxidative process. i.e. you need oxygen for that.

I don't know about any literature that disputes these facts.

Moderate long duration exercise is the most efficient way to "burn" the fat reserves we all want to get rid of. That is physiologically undisputed. And it is the reason why I told Carry not to try to race as fast as she can but start riding easy and long distances. Or walking brisk, but not run for those who prefer that kind of exercise. Heart rate way below your anaerobic threshold.

I don't mean to correct you.
Just set some facts straight. When it comes to giving someone advise how to loose weight that advise should be scientifically sound and based on facts. I think.

It is very posible to do 11 km in 38 min. I just got back rom 13.5 km in 35 min. I am not a super fit nor fast or experienced rider. Go for it girl