Doing two double days to reach 80 miles a week, is more effective for most people. Reasons.
1. You do not have to run long runs every day of 10 miles or so.
2. Doing shorter runs will aid recovery. Example doing two five mile runs a day on two days a week, will feel easier than doing a single "ten" miles.
3. You can use double days to introduce "tempo" into one of the runs on the double days. "Tempo" running is one the best ways to improve performance in distance running, and is not as time consuming or monotonous as track training.
4. People who run a couple of "doubles" a week, obtain a psychological boost from knowing they are training twice a day compared with others who only train once a day. The boost can help performance, because running has a large psychological component.
5. Double days are also good for physiological reasons, because if you run before breakfast your body learns to run while breaking down glycogen and fat reserves....that will help you when you race in a similar situation.
6. Research shows that doing doubles lessens the risk of injuries, occuring from doing all your runs in singles.
My advice - do "double days - 2-3 times a week. The results will usually speak for themselves. Ritzenheim has recently said that doubles have helped him reach a new level, and that one of the reasons he was often injured at Colorado was because he was only doing singles, going long and hard every day.