There's really not much you can do in the two weeks before a marathon to improve your performance, but there is a lot you can do to make your performance worse. It is much easier to maintain fitness than it is to gain it. You can drop your mileage by 50% and still maintain your aerobic fitness. Muscle damage from a long run (2 hours plus) can still negatively affect your running weeks afterwards. So, why are there elite marathons who run 18 milers a week before there race or half-marathon races a week before their race. Simply because over time they have trained their bodies to recovery far more quickly than the average runner.
If you're a first time marathoner it's much better to err on the side of caution. Meaning, don't run anything longer than 2 hours 3-4 weeks out, don't race within 2-3 weeks, nothing but short easy runs the week of. All your training for a marathon was done months in advance. The last month leading up to a marathon should be about building confidence in the race pace and about being light and ready to race. So in that last month a couple times a week go do something at planned marathon pace plus something short and sweet to maintain the zip in your legs.