Running as a popular and beneficial exercise:

Being physically active has never been more important in an era of ever-increasing lifestyle diseases like diabetes, obesity, and cardiovascular illnesses. Running as a physical exercise to counter a sedentary lifestyle is popular among many due to its inherent simplicity. One simply requires a pair of shoes, an open space, like a beach or a road, and a willingness to get up from one’s bed. Recent trends in events, like marathons, attribute to the ever growing popularity of running as an exercise to keep one physically active and fit. Regular runners are less prone to type 2 diabetes, heart disease, and even stroke. It also keeps their body in shape and helps strengthen the bones and muscles. Another popular benefit associated with running is the enhancement of one’s mental health state.

Potential limitations of excessive exercise:

A 2012 report published in Medicine & Science in Sports & Exercise journal, based on an observational study conducted on 52000 adults over a 15 year period, informs that runners (who ran 1-20 miles per week at 6-7 miles per hour speeds, 2-5 times every week) had 19% reduced risk of ‘all-cause’ (from various factors) mortality in comparison to non-runners. However, such an association was not observed when the distances ran, the frequency of running, and speeds increased (i.e. over 20 miles per week at 8 miles per hour). Regular marathoners (who run over 25 miles at a stretch) are at an increased risk of heart disease, says a German study.

Also, despite instances of runners exhibiting longevity and improved fitness levels, there appears to be doubts about whether extreme levels of exercising can actually grant higher physical and physiological benefits. A research study by James H. O'Keefe of the Mid America Heart Institute of Saint Luke's Hospital of Kansas City proposes that there could be potential structural changes to the arteries due to excessive endurance training. They suggest that this may actually reduce the benefits obtained from regular exercising since the body is put under so much strain that its inherent recovery systems seem unable to cope with such excesses. In the long run, the structural changes in the arteries may lead to lowered levels of physical fitness.

Many similar research studies propose similar consequences, though they cite the lack of more evidence, like a lack of large group studies, to actually conclude that excessive physical exercise, including running, can definitively cause long-lasting physical damage or conclusively lowered fitness levels.

Exercise in moderation is definitely not bad:

These evidences do not necessarily indicate that the amount you run must be reduced. It would definitely be safe to interpret from such research that doing regular exercises, including running at individually comfortable levels, will not cause any adverse effect on the body. Of course, the aspect of too much running being harmful or beneficial could have been derived from the many myths caused by popular opinion freely accessible on the internet, as opposed to empirically tested scientific evidence. But how much running is too much? 

Research shows that the ideal running distances are distances that you set and are comfortable with from time to time. The greatest health benefits, though, occur when you run between 5-19 miles a week at speeds averaging around 6-7 miles per hour. You may plan to cover the distances over 3-5 running sessions.

Also, running as a physical activity should not be taken in isolation. An optimal planned diet (not based on fad diets), adequate rest for the body to recuperate from physical activity, and understanding one's own limits in exercising will potentially enable one to derive the maximum benefits from any physical activity. As each individual is different, chalking out a self-optimized exercise plan is one of the better ways to go about avoiding any risks associated with over-exercising or running too much.

References:

Lee J., Patte R., Lavie C.J., Blair S.N. Running and all-cause mortality risk: is more better? Med Sci Sports Exerc. 2012;44(6):990–994.

http://www.mentalhealth.org.uk/help-information/mental-health-a-z/E/exercise-mental-health/ (accessed on 01/22//2015)

O'Keefe, J. H., Patil, H. R., Lavie, C. J., Magalski, A., Vogel, R. A., & McCullough, P. A. (2012, June). Potential adverse cardiovascular effects from excessive endurance exercise. In Mayo Clinic Proceedings (Vol. 87, No. 6, pp. 587-595). Elsevier.

La Gerche, A., Connelly, K. A., Mooney, D. J., MacIsaac, A. I., & Prior, D. L. (2008). Biochemical and functional abnormalities of left and right ventricular function after ultra-endurance exercise. Heart, 94(7), 860-866.

La Gerche, A., Burns, A. T., Mooney, D. J., Inder, W. J., Taylor, A. J., Bogaert, J., ... & Prior, D. L. (2012). Exercise-induced right ventricular dysfunction and structural remodelling in endurance athletes. European heart journal, 33(8), 998-1006.

http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Running_and_jogging?open (accessed on 01/22//2015)

Helpful Peer-Reviewed Medical Articles:

Johnston, C. A. M., Taunton, J. E., Lloyd-Smith, D. R., & McKenzie, D. C. (2003). Preventing running injuries. Practical approach for family doctors. Canadian family physician, 49(9), 1101-1109.

Shorten, M. R. (2000). Running shoe design: protection and performance. Marathon medicine, 159-169.