For some reason people seem to have the idea that running will create fat loss. There’s two ideas that probably contribute to this: the best runners are usually very lean, and the 1980s obsession with aerobics. The latter especially created a tradition so that “everybody knows” you go run or do aerobics classes to get in shape. Right?
As usual, it’s not that easy. There is some truth to the fact that steady-state aerobic exercise does tend to preferentially use circulating triglycerides as a fuel source (body fat, effectively). However as the current fad towards interval training will quickly remind us, the fuel source isn’t the entire story. There’s something to be said for shorter, more intense exercise. I won’t go into the details because they aren’t terribly important here, but maximal activation of anaerobic metabolism tends to chew up more calories and do favorable things for nutrient partitioning: calories go to muscle instead of fat storage, and more fat ends up being burned over a 24-hour period.