The primary source of fuel on the long slow rides are fat, you're right. However, it's rare that any "non atkins" person will burn body fat. Your body will burn the dietary fat first, I believe. Again, I'm no expert, but I've heard it comes down to calories. I could be wrong, but everything I've read for losing weight, lately, suggests intervals. I'll agree that long easy efforts have their place, and they're great calorie burners.