Dress Rehersal - the importance of a practice run

In my experience at putting myself through the pain (and ecstasy) of a marathon training programme and marathon itself, I have often found that doing a 10k, 10 miler or half marathon in the run up, hugely beneficial. So much so, that I often feel I learn more about how my prep is going on a "mini-race" day than on a long run. The practice run gives you the chance to experience the buzz of the race without the pressure that marathon day inevitably brings! You can practice that pre-marathon routine involving what to eat before the race, having gels and water ready, gear laid out etc and see over the course of this practice race day what works and doesn't work for you.
It's also a good chance to overcome nerves and get used to race surroundings with other runners around you rather than those quiet miles you have faced on country roads without any of the bother. Familiarity with these surroundings will help ease real race day nerves when the big day finally arrives.
The best time to attempt these lower mile races is on an easy week with lower mileage when you may miss (or not!) the intensity of that high mileage run.
A strong race at a shorter distance can have postive effects both physically and mentally but it is the practice ritual of race day that  you will feel the real benefits of as you draw closer to that hard earned place in the starting lineup on marathon day.