At first, I answer to the poster that asked "where did you copy these information". I copied from the book that I wrote for IAAF, "Marathon Training : A scientific Approach", that you can have writing to IAAF (I think that the cost is 8 ?, but I'm not sure). So, I copied from myself.
About periodization, I want to weigh up the difference of phylosophy between the FUNDAMENTAL PERIOD and the SPECIFIC PERIOD.
During the FUNDAMENTAL (or GENERAL) PERIOD, our goal is to improve all our qualities (general resistance, strenght, rapidity, flexibility, elasticity, ecc.). During this period, is very important THE INTERNAL LOAD. So, if for ex. I have in my program to run 10 x 1000m in 3 min rec. 1'30", but because of the cold or some personal problem due to my job o my family, I'm not able running under 3'08" (not because I'm lazy, but because I'm not able in spite of my commitment), I have to continue training with the same level of effort, AS THE PHYSIOLOGICAL RESULT IS THE SAME, because the Interanl Load is the same. You have to remember that TRAINING IS THE ANSWER TO A PROPOSAL, NOT THE PROPOSAL. So, if 3 athletes do the same work, we can be sure that really are 3 different level of training, because THE INTERNAL LOADS ARE DIFFERENT. Internal Load is subjective, depending on many factors.
During the SPECIFIC PERIOD, our goal is to use our consolidated qualities for preparing our performance. In this case, is essential the EXTERNAL LOAD. In other words, if I want running 10000m in 30' and I need running 10 x 1000m in 2'57" with 1'30" of interval, when I'm able running in 3'05" because I'm tired or I have some problem, I have to stop training, postponing the session to another day, BECAUSE THE SPECIFIC WORKOUT IS MATHEMATIC : 10 times 3 min are 30 min, and if you are not able running 3 times 3 min, you are not ready for running in 30'. External Load is objective, beeing the same for everyone.
Now, I can continue my explanation quoting myself.