Most exercise advice is based on the minimum necessary to achieve improvement in fitness. This long term study showed that the greater the volume of vigorous exercise, the lower the BMI and waist circumference.
Much of the improvement is probably achieved at lower volumes. For instance, you get more benefit going from 0 to 10 miles a week than you do going from 10 to 20 miles per week. In the end though, the higher volume trainer will achieve better results more often than not.