After all your hard training, remember that you need to give your body a rest in order to aid recovery.  I hear too many stories where runners become exhausted, injured, and demotivated as a result of overtraining.  These people are missing a vital aspect of training effectively - that recovery is just as essential as the hard work!

During recovery time, your body changes - it is repairing muscle tissue and making necessary adaptions to be able to take on the next challenge you throw at it.  It is better to intersperse recovery time into your training week, rather than train for 10 days solid then have 3 days off.  And a rest day doesn't necessarily mean doing nothing - low level exercise can improve your recovery.  Yoga, pilates, stretching or just a good old walk are all great recovery activities.