Exercise does not make you fit It is the rest that follows exercise that makes you fit.
While this may seem like playing with semantics, it is vital to grasp this concept. This is how a single bout of training impacts your fitness: Note that the training reduces fitness, and that rest time is requied to gain the benefits.
1 Adequate Rest
Ideally, you will get sufficient rest so that you recover from the training stress and get the maximum benefit. This would mean starting the next training session near the peak of supercompensation, as shown below. (The image is quite wide, and may not show up fully on a low resoution screen - there is a scroll bar at the very end of the article, or view)
2 Insufficient rest resulting in stagnation
Without sufficient rest, you may start your next training at the end of the period marked ‘Recovery’. This means that you have not benefited from the training, just barely recovered from it. This can carry on indefinitely, with no improvement in fitness. (View image)
3 Insufficient rest resulting in injury
Without even sufficient rest to recover from the training stress, the fitness level starts to decline, with each training bout further reducing fitness. Sadly, the response to this is often to increase the training stress, creating a positive feedback cycle. Eventually this will result in some type of injury.
4 How long to rest?
I am a big believer in running four days a week, as it allows for tough training and good recovery. However, different levels of training stress will require different rest periods. To make matters more complex, different systems (muscles, cardiovascular, tendons, bones, hormones) are likely to require different rest for a given training stress. This means that the optimum amount of rest is likely to vary significantly. However, it is generally accepted that doing ‘quality’ or ‘hard’ workout every day is too much. The idea of alternating hard and easy days is based on the need for more than 24 hours rest. I believe it is reasonable to conclude that the ‘easy’ days do nothing more than interfere with the rest that is needed, and hinder fitness, rather than improving it. Hence the idea of training four days a week.
5 Too much rest?
It is possible to get too much rest. Trying to gain fitness on one or two days training per week does not seem to work for most people.
(Original image is copy-left from http://en.wikipedia.org/wiki/Supercompensation)