"Many runners introduce two runs per day before it is necessary. If your are preparing for races of 10,000 meters or longer, avoid double workouts until you have maximized the mileage that you can positively recover from in single workouts. Staying with longer single runs builds endurance and gives you more time for recovery between training sessions.Later, he affirms that
"When your mileage increases to the point at which your recovery runs last more than 50 minutes (or more than an hour during high mileage marathon training), then it is time to switch those days to easy double-workout days. Doing two runs of 35 minutes rather than one 70-minute run is easier on your body and enhances your recovery.