How Often Should You Run?
As with every training program, the answer depends mostly on yourself. However, if you'd like to make progress, practically speaking, you should run at least three times a week. Some elite and professional runners run up to twice daily, in 10 mile sessions or longer. There is a wealth of research which, not surprisingly, tends to follow the conventional wisdom that working out as much as you can manage tends to yield much better results. While it is certainly true that doing moderate exercise daily (yoga, swimming, pilates, weight training, ...) will return great benefit, research indicates that pushing your limit consistently is the best way to make tangible gains. Of course, it's not always everyone's goal to become the most successful athlete of all time. Clearly, even doing light exercise such as walking or tai chi a couple times a week cultivates a stronger and more healthy body, it should not in any case be ignored.

The short answer to this question is that you should run as often as you can bear; in the beginning however, it's important to avoid injury. The stabilizing muscles in the knees are particularly weak in the untrained, and prone to injury after very little stress. If you sense pain in your knees, it's a good indication that you aught to rest an extra day or two. A study conducted on untrained athletes shows that the short term gains in the early stages of running at high intensity for periods longer than 45 minutes more frequently than 3 to 5 times per week are outweighed by the incidence of injury to the knees caused by over-training. It may help you to know that this state is not permanent, as the muscles around your knees are developed and strengthened. If you'd really like to continue training during the down time of more high impact exercise, yoga and swimming are a great alternative that will make your heart as happy and healthy as you deserve. Keep at it, and live long.