We have been doing a heap more running than usual at Hybrid in preparation for Tough Mudder. Normally we do not prescribe long distance running. This is for a few reasons, it is catabolic (i.e. eats our muscle away), it is hard on the joints (I guarentee pretty much every long distance runner has pain issues of some sort), and it encourages the body to actually store fat. Compare a marathon runner (no muscle, slight build but still a high fat percentage, against a sprinter who is lean, has good muscle mass, and no body fat)
Yes it is good cardio but the benefits in long distance running do not outweigh the negatives. Normally, if you are not training to do a marathon, half marathon etc I would limit your running to sprints and at the most only one 5km run per week.
For those that do run the following graph is for you. I also know the reason you run is probably for personal achievement and accomplishment and you are training towards a particular event. For those that run for the sake of fitness then slow down, get into the gym and do some strength work or some metabolic conditioning. Maintain and build muscle whilst shredding fat.
Strength is the most important component of fitness, only the strong will survive. Every other component of fitness is reliant on strength, all you runners out there hit the resistance training.
We will still be doing longer runs over the next few weeks only to prepare us for Tough Mudder where we have to cover 19km. The below picture is an infograph on American runners. Interesting reading though. A big increase in running is being witnessed. Keep on running but go faster over less distance for better results and fitness.