Excerpt from about.com...
Mondays and Fridays: Mondays and Fridays are rest days. Rest is critical to your recovery and injury prevention efforts, so don't skip your rest days. You'll also get mentally burned out if you run every day with no breaks.
Tuesdays, Thursdays, and Saturdays: After you warm up, run at a comfortable pace for the designated mileage. Make sure you cool down and stretch after your run.
Each week, you'll increase your runs by a quarter mile, which is a
lap on most outdoor tracks. If you usually run on roads and you're not
sure how far you run, you can figure out the mileage by using MapMyRun.com. Or, you could drive your route in your car and measure the mileage using your car odometer.
Wednesdays: Do a cross-training (CT) activity (biking, swimming, elliptical trainer, or other cardio activity) at easy to moderate effort for 30 to 40 minutes. Strength-training is also very beneficial for runners. If you're feeling very sluggish or sore, take a rest day.
Sundays: This is an active recovery day. Your run should be at an easy, comfortable pace. Or, you can do a run/walk combination or cross-train (CT).
can switch days to accommodate your schedule. So if you're busy on
another day and prefer to workout on a Monday or Friday, it's fine to
swap a rest day for a run day.