The idea is to build your mileage slowly. I run my short-med runs on Tue-Thu-Sat (8-14km), with my long run on Sundays (12km-36km), building on a 5 week cycle of 4 weeks increase-1 week easy.
You can find a few good programs on the net. There's a good one here -
http://www.teamoregon.com/publicatio...at.html#ONLINE