Here’s an idea that has worked well for me when it comes to running – limit your running to 3 times (no more, no less) a week. Running puts a significant stress on the body and the more you run, the higher the risk of injury, especially the more days you run. Running 3 times a week allows at least one full recovery day between runs and puts more spring and freshness in your legs than running every day. From my anecdotal observations and experience, going beyond 3 days a week is just not worth the heightened risk of injury. Assuming you are not an elite athlete, you can probably accomplish your running goals if you ensure each run is of quality and has purpose – an interval run for speed, a tempo run for stamina, and a long run for endurance. However, I would suggest on the days you don’t run, do some cross-training such as biking, elliptical, swimming or rowing. The cross-training not only helps your aerobic capacity, but also develops other muscles that may further help prevent injury.
I was able to qualify for the Boston Marathon running only 3 times a week, so I know this is an effective method for some people as long as you are focused on quality rather than quantity. If you want studies to back this up, I suggest you take a look at FIRST training program web site.