Base Training - My Understanding

My understanding of base training is that you train at a relatively easy pace in order to improve particular body systems. Keeping your heart rate within a certain range typically of around 65% to 75% of maximum for 90 mins or more each time is said to improve aerobic capacity, endurance capability, promotes fat burning for fuel instead of glycogen/carbs and provides a firm foundation from which to launch harder and faster efforts later in the year. Base training stimulates slow twitch muscle fibres (endurance) and enables them to work using less oxygen from the blood. This type of training is also said to build the heart muscle enabling more oxygen rich blood to be pumped to the muscles with every beat. Other claimed advantages are that you will extend your time to glycogen depletion by as much as 75% meaning you can eventually train harder and faster for longer before you 'bonk' or 'hit the wall'. 'Bonking' happens when your body has used all its stores of fuel that it can convert quickly to energy, namely glycogen. Your body then switches to burning fat which takes longer to convert to energy so as a result your body makes you slow right down while it does this - the 'bonk'.
There are numerous schools of thought on how long to perform base training before ramping up the effort to improve your speed. CyclingInform.com states that 2 months is about right, which is probably around 1000 miles. Cptips.com quotes a 500 mile base training period http://www.cptips.com/preseas.htm.
 All of them say to revert to base training again at the end of the season, typically winter months in order to see continual improvement over the coming months and years. This is because our bodies have an amazing capacity to adapt to the training stresses we impose, and so if we train the same way all the time, our bodies adapt and we don’t see continued improvement.

Further reading on base training is an excellent book by Thomas Chapple, available from Amazon for £13.50.