Feeling the burn after a workout is a great sign that you've done your job. That burn is a result of pushing your body past what it's used to. Challenging yourself toward harder, more intense workouts over a period of time is called progressive overload. Progressive overload trains your body to adapt to the new conditions being put upon it.
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The key, however, is making sure that along with the progressive overload you are also giving your body time to recover. Ever notice how most marathon training plans have you run a 20-miler followed by a day of rest and a shorter "long run" the following week? Or the plan may have you increase your long run mileage a little each week up to a certain point and then drop off and build back up again.
That's progressive overload or stress adaptation. Build up. Back off a little. Build up. Back off a little. Overloading the body and then giving it a chance to recover, adapt, and heal before placing more stress upon it, is a great way to train.
New runners and seasoned runners both can get caught up in the excitement of training. Before they know it, they've peaked, burned themselves out (physically and/or mentally) or worse, suffered an injury before race day. Below are some good tips to follow to get the most out of your training without overtraining.
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1. Follow the 90 percent rule. When doing quality workouts (hill repeats, tempo runs, intervals, long runs), push yourself, but always leave something left in the tank. Think about pushing yourself up to about 90 percent of your maximum effort, but never give push it to maximum effort. After finishing a quality workout, you should feel tired. You should feel like you've worked hard, but you should also feel like, "Hmmm, I could have done a little more." This should be a good feeling, not something to beat yourself up about. Knowing that you've worked hard (close to maximum effort), but not crawling away from the workout and needing three days to recover will greatly benefit you in the long run. Doing every quality workout at maximum effort is an injury waiting to happen. Just knowing that you have that "extra" in you can really help you beat mental and physical fatigue later in a race.
2. Enjoy the easy runs. Almost every training plan includes easy runs each week. These runs are usually designed to keep your base mileage going and to help keep you limber in between quality workouts. Problem is many runners blast through the easy weekly runs as if they were quality workouts. If every run is a hard run, you greatly increase your chances of injury, peaking early, or mentally burning out. Also, running your weekly easy runs at maximum effort can put a damper on your weekly quality workouts and so they're not benefiting you as much as they should.
3. Respect your REST days. Forget the idea that rest is only for the weak. Rest is equally as important as that weekly tempo run or long run. Your body needs time to rebuild the muscle tissue that's broken down with each workout. If you never let your body rest, your fitness level can begin to decline affecting all of your runs, easy and/or quality. No rest is basically a fast forward to overtraining and injury.
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4. Follow the hard/easy rule. Overtraining can be a result of repetitive exercise. If you don't vary your workouts and you're constantly subjecting your body to the same stress over and over, those muscles can become over-trained. A good rule of thumb is to wait at least 48 hours before working the same muscle groups again. So for example, if your do a chest/triceps workout one day, you should wait at least two days before working those muscles again. Professional bodybuilders will often work a muscle group so hard in one workout, that they'll wait an entire week before working that muscle group again.
In running, you should think more in terms of hard/easy. Hard (or quality) workouts can include speed workouts (intervals, repeats, tempo runs, hill repeats) that are run at about 80 to 90 percent of your VO2Max. Hard runs also include the weekly long run which is run at about 60 to75 percent of your VO2Max. Even though the long run is an easier pace, it's considered a "hard-run" because of the distance. Easy workouts include short or mid-distance runs that are run at an easy to moderate intensity (60 to 75 percent of your VO2Max). So, if you do a hard run one day you should wait at least two days before running your next hard run.