The body does not have a chance to rebuild in response to overload in such situations. Hard can also be defined in terms of distance or pace. In the sharpening phase, hard workouts will use increased intensity or pace. The basic training for all distance racing is endurance training. The runner must have the stamina to cover the desired distance.
Cardiovascular endurance comes first.
Then the specific muscles become stronger, followed by the connective tissues, tendons and ligaments. Injury often occurs because the runner wrongly feels that he has the stamina to run the required number of miles, but actually lacks the muscular and connective tissue strength which develops much more slowly. Base building for the marathon should follow a schedule designed to build mileage slowly and comfortably to the level where your body can tolerate the necessary long training runs. The normalized distances are based on the hard runs being either 2 or 3 times the distance of the easy runs. This results in maximum improvements in aerobic metabolism which lead to increased aerobic enzyme production, better fat utilization and adaption of convertible muscle to aerobic use.