Improvement in physical fitness results will only occur when specific muscles and systems of the runner are stressed and challenged by a workload greater than normal.
The overload of the system must occur through a gradual build up in duration and intensity. A safe increase for running is 10 per cent per week in additional overload.
Adaptations to training of muscles and systems is best achieved when the runner adds quality running to the program. As an example, strength training is best done by running hills. Cycling or cross-training in the gym will help, but the best strength work comes from specifically overloading the muscles and systems used to run and to do the overload through running.
Exercise stress on the body of the runner dictates the need for rest. Rest is a good four letter word that allows our body systems and muscles to rebuild and get stronger. Failure to rest will result in overtraining, which can lead to fatigue and injury. Walking is a good way for the runner to rest.
Too much rest can result in a loss of running fitness. Loss of muscle strength is fairly slow. Loss of aerobic fitness is fast. It generally takes three times the amount of time to lose fitness as it does to gain. This is why consistency of training is vital to the success of your running program.