Long Slow--continuing documentation.

A study published in The Journal of Strength and Conditioning Research once again validates the maximum fat oxidation that happens with low-exertion exercise.

Here's the crux:

If laboratory testing is not possible, the (subject) can reasonably confident the MFO lies between 60.2% and 80.0% of the maximal heart rate.

“It’s probably best to work out near the top of that zone,” the study's author says, “so that you burn more calories over all” than at the extremely leisurely lower end.

Other research summarized in the New York Times addresses another urban myth. There's no afterburn effect of exercise.