You've Increased Mileage and Speed Simultaneously
Even though you may be following a sensible program to increase weekly mileage and speed, doing both at the same time can place too high a demand on the body, leading to injury. Technically, this is overtraining, but you're just not thinking of it that way because you've tried to make a point of doing things right. Many runners can get away with very gradual increases in both, but brand new runners, heavier runners and older runners are more susceptible to this type of training injury. Play it safe by building up mileage first, then start training for speed.
Learn More: 6 Tips for Heavier Runners
You Refuse to Lift
Don't be one of those runners who never does anything but run! Weak core muscles--glutes, hips, abs and lower back--can contribute to poor form and, ultimately, injury. Work with a coach to add some runner-specific strength training to your schedule two or three days each week, and consider having a professional assess your running posture and gait.
You've Added Too Much Strength
A little strength training is great for runners, but too much can stress the muscles and the central nervous system, leaving you vulnerable to injury. If you find yourself working out six or seven days a week or running and lifting on the same day more than once per week, this could be why you are injured. Even if you only strength train one or two days per week, going heavy or doing energy-demanding lifts at the peak of your running training (in the middle of race season, for example) is just another version of overtraining. Lighten your load by putting limits on your heavy strength training.
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