It's easy to get injured; anyone can do it. Just run too much. "I firmly believe that every runner has an injury threshold," says physical therapist and biomechanist Irene Davis, Ph.D., from the University of Delaware's Running Injury Clinic. "Your threshold could be at 10 miles a week, or 100, but once you exceed it, you get injured." Various studies have identified injury-thresholds at 11, 25, and 40 miles per week. Your threshold is waiting for you to discover it.

Of course, your goal is to avoid injury. Runner and sports podiatrist Stephen Pribut, D.P.M., warns runners to beware the "terrible toos"-doing too much, too soon, too fast. Every research paper and every expert agrees that this-"training errors"-is the number one cause of self-inflicted running injuries. The body needs time to adapt from training changes and jumps in mileage or intensity. Muscles and joints need recovery time so they can recover and handle more training demands. If you rush that process, you could break down rather than build up.

Running experts have recognized this problem, and long ago devised an easy-to-use 10-percent rule: Build your weekly training mileage by no more than 10 percent per week. If you run 10 miles the first week, do just 11 miles the second week, 12 miles the third week, and so on.

Yet, there may be times when even a modest 10 percent increase proves too much. Biomechanist Reed Ferber, Ph.D., an assistant professor in the faculty of kinesiology and head of the Running Injury Clinic at the University of Calgary says that he sees a lot of newly injured runners during that third month of marathon training, when a popular 16-week Canadian program pushes the mileage hard. Meanwhile, his clinic's nine-month marathon program for first-timers increases mileage by just three percent per week. "We have a 97 percent success rate getting people through the entire program and to the marathon finish line," Ferber says.

ACTION PLAN
Be the Tortoise, not the Hare. Increase your weekly and monthly running totals gradually. Use the 10-percent rule as a general guideline, but realize that it might be too aggressive for you-especially if you are injury-prone. A five-percent or three-percent increase might be more appropriate. In addition to following a hard-day/easy-day approach, or more likely a hard/easy/easy pattern, many top runners use a system where they scale back their weekly mileage by 20 to 40 percent on a regular basis, maybe once a month. And remember that mileage isn't the only issue. Experts point out that an overly aggressive approach to hill running, intervals, trail running-indeed, any change in your training habits-can produce problems. Keeping a detailed training log can help you gauge your personal training threshold. Record your weekly mileage and how you feel after your runs. Look for patterns. For instance, you may notice that your knees ache only when you're logging more than 40 miles a week.

.Another major bugaboo: You used to run 30 miles a week, you got injured, now you want to get back to your old routine as quickly as possible. Don't. Take your time. The same applies to that upcoming race-if you missed some training time, don't accelerate the pace and distance of your remaining workouts in an effort to "catch up." Instead, adjust your goals as needed.
 

I would add a caveat here that you should pay attention to your MENTAL limits as well as PHYSICAL limits.  It is possible to get psychologically exhausted BEFORE you reach your physical limits.  That could lead to a different kind of "injury", but no less hazardous to reaching your goals