found this on page 49 of runners world magazine november 2006...

"studies have confirmed that high carb diets enhance endurance by enlarging glycogen stores. in one study, runners eating a diet high in fat and protein and low in carb had a maximum endurance time of 57 minutes.

those eating a diet of about 50 percent of calories from carbs lasted about 114 minutes; and those eating a high carb diet (83% of calories) were able to run for 167 minutes"

makes that green banana smoothie with extra goji and dates all that bit sweeter! lol!