Sometimes it's tough to take a day off. Especially when you're in a good rhythm and working toward lofty mileage goals. But rest is a necessary "evil," and I encourage you to build it into your training plan. And I'll try to do the same!
See, so far in 2012 I've gone on a run every day except January 29, when I was struggling with some knee tendinitis.
On one hand, it feels pretty good for my ego to have been so consistent. On the other, I'm certain I'm straining my legs a little too much when I should be taking a day here or there.
What should we do with our down days? I don't think it hurts to sit around and be a lazy sack. There was a successful female runner who relatively recently said during an interview that she takes completely down days where at most she simply stretches for a few minutes. You could also do a little cross training if you like, but go easy, my friend. Don't be an excercise-aholic. Life's too short to be an addict.
The reward for deliberate rest is fresher legs and prolonged health. Though it isn't possible to be scientifically sure, I'm 100% confident you will ultimately be faster if you take a day off now and again.
Happy running and resting,