*It is always best to consult a physician before preceding with any treatment plan.*
When people think of runners, they get this notion of people who are constantly just running. 3, 5, 10, 20 miles a day of just running. The fact of the matter is that the best runners out there are very strong athletes. What this means is it is important as a runner to not neglect strength training and variety in ones workout routine. In order to maximize your fitness variety is vital.
Strength training and cross training (pretty much any exercise that is other than your main activity), works to develop your support muscles, gives your nervous system a different stimulus, all while staving off plateaus mentally and physically. When you run you are using a certain set of muscles over and over. These muscles become fatigued and your body starts to recruit other muscles to take force off of these fatiguing muscles. However, if one does not focus on strengthening these support group muscles they can not be used when they need to be called upon. As one can imagine this leaves the work up to the already fatigued running muscles and you become more susceptible to injury. Also, varying your routine keeps things fresh. You can look forward to cycling, rowing, swimming, rock climbing, snow boarding, skiing, boot camp, stand up paddle boarding, and then running. By incorporating so much variety you appreciate each workout a bit more and YOU FEEL STRONGER.
Another important reason to cross train other than staying injury free is that the body responds to variety of stress more effectively. If one is to do the same exercise day in and day out without variety in the distance or the intensity, the body eventually becomes accustomed to this routine and does not build upon its previous level. This having to continually differ the stress in order to improve is known as the "General Adaptation Syndrome".
The best visual of this phenomena I can find is pictured below:
So go ahead and mix things up. Your body will thank you with feeling better and stronger.
Some Good Cross Training Options: