Our body reacts to strenuous activity like distance running in two ways: acute reaction and training effect. Acute reaction is when your heart rate speeds up, your stroke volume increases, your ventilation rate and depth of breathing increase, your blood pressure rises, and your muscles feel some fatigue. However, when an activity becomes regular routine to you, you will eventually feel the second reaction which is the training effect.
It is when your body gets used to chronic exercise; your muscles feel stronger and less discomfort to physical activity. Through training, your blood flow increases that it allows you to produce more energy and less lactic acid to accumulate during exercise. Your resting heart rate becomes slower caused by a stronger heart that can pump more blood per beat. You will also most likely develop a lighter, springier step, lower resting blood pressure, lower body weight, and less fat under the skin.
Your body now can tolerate stress and ready for competitive efforts. "The system you stress during exercise is the one that stands to benefit from the stress," wrote Jack Daniels in his book Daniels' Running Formula. However, the principle of specificity restricts you to achieve gains through performing another activity. By doing another activity, you take yourself away from your primary interest and might produce results that limit performance in your main activity. Long distance running has negative effect on performing explosive leg activities like sprinting and jumping. Thus, you must give considerable thought to every aspect of your training and you must keep in mind that everything you do affects you and your body's reaction.
Adding a new level of stress on top of your current training further increases your fitness level. In distance running, if you have performed a training routine regularly and reached a stable level of proficiency, you can possibly take more training modifications (for frequency, duration, intensity, or recovery) which will lead you to a new level of fitness. For example: you can increase the training frequency from three to four days per week, you can increase the amount of training from three to four miles per session, or you can increase the distance of each interval from one mile to one half and a half each. Another possible modification is to change the recovery time allowed between the mile runs within a workout.
Sticking with a training program for longer than six weeks produces more benefits. If you want to increase training, the best time is after six weeks of adapting to a specific training stress. The primary risk of increasing training too often and too soon is an escalated risk of injury and overstress. There are limits to training for distance running.
Overstressed system can affect variety of activities other than just the activity that caused the damage. For example, a stress fracture in leg caused by too much running or improper running can restrict a runner from doing other activities that stress the injured extremity. Thus, it is important to watch out for abuses that you can possibly do as you try to reach your goals. You must learn to accept that even you are physically active; your body has still limits that dictate your success.
In distance running, few people realize their limits and improvements are always possible. However, keep in mind to take things at the perfect timing.