most of the fat people you see at the gym are running too slow to do them any benefit. they think they need to be in the FAT burning zone, (ie 50% of max heart rate) but that by running or working out in that zone you are burning less total fat than if you were working out at say 70-80% of your max heart rate. But even that isn't enough, the body needs more stimulus than that to really make a change.
I had great success with a girl who thought working out meant running in that 50% zone and doing yoga. I started giving her workouts, 2 hard running workouts a week, one more aerobic in nature and the other more anaerobic, with 3 other days of 30-45 mins at 70-80% of max heart rate. She still did Yoga 2-3 times a week, she lost 15 pounds of fat, and became a pretty good runner.
I agree that just running slow isn't going to do you much benefit, but if you train someone like an actual runner and that doesn't mean 100 miles a week, 20-30 miles is enough with a couple hard workouts a week. you'll get results with anyone and they'll still have enough energy to lift weights or do other workouts if they want to.