Originally Posted by
runnercycler
I would say 20 mi is a bit long for most milers. I am a 5000 meter runner and only have a long run of about 16 mi which I would say if fairly typical. Most collegiate milers are going to be in the 13-16 mi range. There are physiological changes that only begin to occur after a certain period of time such as increasing the number of mitochondria and capillaries in the muscle. This improves their oxygen transport efficiency and is highly beneficial even in events as short as 800m. More aerobic work would be better but running for two hours is a lot tougher on the body than cycling for two hours.
This is "raise the left". You want to force your body to adapt an increase mitochondira density/function. IE sprint for 3 minutes in a hard > threshold vO2 effort, this forces your body to adapt to the hard effort, which in turn makes the 60-70% effort of long rides easier to maintain.
It's the basis for interval training.