In reply to BusyLizzie:
Based on someone running regularly and reasonable mileage :-
This week, ie 3 weeks to go drop your volume to 70% of what your average has been last few weeks ( and a long run of about 15 next weekend ).
Next week, ie 2 weeks to go drop to 50% of your average ( and long run of 13 miles 7 days before )
Final week, drop mileage to about 30%
This week and next week when dropping mileage, still stick to same number of days per week, but cut how far you run each day ( rather than keeping distance the same and cutting out days ). Keep intensity the same, don't just do easy recovery miles for every run.
Final week have some rest days and all runs nice and easy.
Diet - just eat sensibly, especially final few days, don't change much ( assuming you are currently eating sensibly !! ).