Coach Tony V talks about Run Frequency:

The message this month is FREQUENCY - specifically RUN FREQUENCY.
I recommend running shorter runs but more often.  In the peak volume for a runner it is not uncommon to do 10-12 run workouts per week.  The difference between the experienced runner and the uneducated is that the uneducated still thinks they need to run 20 miles straight to be able to survive a marathon.  The experienced runner knows that after putting in a 50 mile week, a marathon is a piece of cake - at least as far as marathons go.  That 50 mile week can be done without a single run over 90 minutes.

Now I'm not suggesting you all go out and run 50 miles per week.  But I am suggesting you all go out and start increasing the frequency of your runs.  If you're already running, simply take your present total weekly run mileage and distribute it over five or six days.  Mix up the distances so that you have some short and some longer days - none over 2 hours.  Many studies show that runs longer than 2 hours elicit very little performance gains over runs of shorter duration while the risk of injury goes up dramatically.  It is better to run 30 minutes twice per day six days per week than to run four hours and have to take three days to recover.  You can tack the easy runs onto the end of bike workouts to save on laundry (then you don't need two sets of workout clothes) and it is better to do your longer runs before bike rides and just spin easily after the run.  If you're running 30 miles per week I suggest you run 3 miles one day, 6 the next, 3 the next, 6 the next, 3 the next, 9 the next, and rest on the 7th day.  That is a good model to follow even if your mileage is different just follow the trend.

If you're not running at all, you should start by running 20-30 minutes at a time every other day.  (If you've never run, you can follow the same plan but walk instead of run until you max out your frequency and duration).  After a week add another day of running.  After a week add another, and another until you can run 5-6 days per week for 20-30 minutes each.  Once the frequency is up then you can increase the duration on alternating days.  Always gradually increase no more than 10% per week.  I recommend running for time and not worry about distance.  I also recommend out and back routes rather than loops as loops can commit you to running further than you intended.  Simply run anywhere you want for half your time then turn around and run back the same way you came.  Once you've maxed out your frequency and duration then you can start increasing intensity.

Remember you do need rest.  DO NOT run seven days in a row.  Be sure to take at least one day of complete rest, and take more if you need it.