i would say to get quicker......it helps, maybe, to get on the hills for some of your longer runs. Running a flat (ish) course marathon will feel a lot less daunting if you have done 18-20 mile hilly training runs. It also helps to do interval training and running outside your normal comfort zone. But fundamentally the faster you want to go, the heavier the mileage needs to be. But with that comes a greater risk of injury, bit of a catch 22. What works for one individual may not be so good for another.