It is measurable, but it's not as easy as in your example. A single 4-5 hour long run burns a pound of fat (and uses many times that much water) for a moderately fit man. If effort isn't made to regain that fat, it's gone. In general, I think an hour for .2lbs of fat is probably a good bet. Running is one of the greatest fat burning exercises there is, and it also protects muscle and bone density in a way that pure dieting won't. But most people vastly, vastly underestimate the amount of work it takes.