Why Slow or Easy Running should form a large percentage of your weekly running
The number of Slow Twitch fibres you have will ultimately define your potential as a marathon runner. Training to improve these slow twitch fibres can have a significant effect on your marathon capability.
Easy running recruits a whole host of slow-twitch fibres because they have a lower contraction or “firing” threshold than the more powerful fast-twitch fibres. Like any other muscle, the more they are used, the more they develop. Along with resistance to fatigue, slow twitch muscles can be relied upon for more miles so that fast twitch fibres are not fully engaged until down the road. In the end easy running helps to develop slow-twitch fibres that are more fatigue resistant than fast twitch fibres.
The more slow-twitch fibres you have, the better you will be prepared to use fat for energy. The body contains copious amounts of fat to burn and only a limited supply of carbohydrate. The greater length of time you burn fats, rather than carbohydrates the longer you put off the glycogen (carbohydrate) depletion and an encounter with the dreaded “wall”.
When you run at lower intensities, you burn somewhere around 70% fat and 30% carbohydrate. With an increase in pace comes an increase in carbohydrate you burn. Your easy running days serve as catalysts to develop those slow-twitch muscle fibres and consequently teach your body to burn fat instead of carbohydrate.
(taken from Hanson's Marathon Method)