Well, I am no biochemist, that's for sure! But from what I have read... for example:

http://en.wikipedia.org/wiki/Cellular_respiration

This article talks about the number of ATP molecules produced per glucose molecule using different energy pathways. "Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism."

What amazes me is how slow I am on a bike. OK, I have a heavy bike, and my BMI is in the 25 zone. But seriously, I keep my heart rate up around 140 - I am 57 yrs old - and I sure am not getting anywhere too fast! Oh well, I enjoy the scenery and it must be keeping me half way healthy, despite all my limitations!