Characteristic Slow Oxidative (SO) or Type I Fibers Fast Oxidative-Glycolytic (FOG) or Type IIa Fibers Fast Glycolytic (FG) or Type IIx Fibers
STRUCTURAL CHARACTERISTICS      
Myoglobin content Large amount Intermediate amount Small amount
Mitochondria Many Many Few
Capillaries Many Many Few
Color Red Pink White (pale)
FUNCTIONAL CHARACTERISTICS      
Capacity for generating ATP and method used High capacity, by aerobic cellular respiration Intermediate capacity, by both aerobic cellular respiration and anaerobic respiration (glycolysis) Low capacity, by anaerobic cellular respiration (glycolysis)
Rate of ATP use Slow Fast Fast
Contraction velocity Slow Intermediate Fast

Muscle Fibers for Endurance Runners

Obviously, the most important muscle fiber type for endurance runners is Slow switch fiber (type I, or Slow oxidative). It is fatigue resistent and very energy efficient.

Adaptation

The FOG (intermediate) muscle fiber can be converted to slow or fast switch muscle fiber depending on the needs and adapatation.

In fact, adaptation can even slowly convert between all three types of fibers (maybe in years).