Being injured set up back on your schedule and can psychologically as well. I've had more than a few injuries and am in fact, being treated for two now.  If you want to avoid these set-backs, please read on. This excellent excerpt reprinted with permission from Human Kinetics of Triathlon Workout Planner by John Mora.

"The biggest key to avoiding injury is really quite simple in concept but much, much harder in execution—high adaptability. What does it mean? Adaptability means that at every stage of your triathlon odyssey, from planning to training to racing, you must be receptive to your body’s signals in order to make quick and timely adjustments.

When setting up your training plan, it’s vital to plan and execute your training defensively. You’ve all heard the term that the safest drivers drive defensively, with a vigilant mindset and the wherewithal to recognize dangerous behavior. If you want to safeguard your body and avoid injury, planning your triathlon training with the same kind of defensive mindset is the single best thing you can do for yourself.

As you sit down to plan your schedule, listen for those instincts that may be telling you you’re overloading yourself or stacking too many workouts on top of each other, which may break down your body. Besides listening to your gut, you can use the following tips for planning your training defensively:

• Plan your running carefully. Unless you are one of the few triathletes blessed with the perfect runner’s body, flawless form, and near-perfect technique, pounding the pavement is the activity (of the three disciplines) with the highest probability of injury. A five-year study by Staffordshire University of 116 triathletes with differing abilities—from elite triathlete to weekend warrior—showed that 58 to 64 percent of all overuse injuries stemmed from running (Vleck and Garbutt 1998). Be very careful when increasing distance or intensity abruptly, without giving your body a chance to adapt. Always plan an easy workout after a high-intensity or long run and never increase your total weekly running distance by more than 10 percent from week to week.
• Lean toward safety. There may be several critical points in your training at which you feel a certain workout may be pushing it or that your body’s ability to safely finish that extra long run or that unusually hilly bike ride is suspect. Although some measure of risk is acceptable, there’s no shame in playing it safe by reducing the intensity or distance (or both) of these demanding workouts. If your gut tells you that you may be skirting that fine line between better performance and injury, err on the side of caution and make the necessary training adjustments. As you sit down to plan or review upcoming training, shave off some distance, notch down the intensity, or consider an easier course if you feel that your body may be on the brink of overuse.
• Avoid the superman syndrome. Anybody who has ever had a sports injury can probably point to a specific workout or a series of efforts over a short period of time that caused it. Unless injury is caused by something traumatic—a fall on the bike, a dog bite on a run—the root cause is usually overuse that can be clearly mapped in a training log. So if it’s so easy to document an injury afterward, why is it so hard to avoid one? Part of the answer is that we become so attached to our training that our self-image and ego become entangled in it, making it hard to accept the blatantly obvious warning signs. This so-called superman mentality can fool you into believing one more track workout or one extra set of ascending laps will do no harm. That’s why reviewing your daily training log entries from the previous weeks and months on a consistent basis is so important—it keeps you in tune with the reality of your recent training. Examining your log can provide valuable evidence that an injury may be imminent, giving you an opportunity to dial down training and allow your body an opportunity to recover from any cumulative muscle tissue damage from several weeks of tough workouts.

During demanding peak training periods, make some time to take a deep breath (literally), step back, and examine your training with an objective eye. Doing so can help you to better see warning signs so that you can adjust your training to avoid injury."