This from a blog I read yesterday and I loved it:
"How often we run, plays a major component on how well our body adapts to the act of running. Unfortunately this major tenet of training has come under attack in recent years by a large number of ‘do more this less’ faux groups. Leaving the staggering physiological evidence out for a moment, you can still begin to get an idea of the importance of frequency in training when you look at the simple differences between a person who does not run at all, the average mid pack half marathon runner and an elite half marathoner.
Person # Of Runs Per Week Half Marathon Average Joe 0 none
Joe Mid Packer 3 to 5 middle of the field
Joe Elite 13 wins
From this basic chart we can a good sense of the importance that frequency must play in improvement in running performance. Simply put, the body adapts amazing well to something it does on a regular basis and rather poorly to something it only does occasionally. But changes in frequency must be made slowly over time."