At a marathon prep lecture last night Bobby McGee (www.bobbymcgee.com) mentioned a "15 mile rule." Basically you can do a training run up to 15 miles without special preparation or recovery and can run another 15 miles the next week without problem. Over 15 miles you need an explicit recovery period and another 15+ run the following week is out. It sounded like time isn't a factor. Some people can do this in 90 minutes and shouldn't run longer, others will take 3 hours and that's not a problem.