The more miles per week you do, the better you will get, but increase gradually. A weekend long run is vital. A midweek shorter/faster session is a good variation.
I used to think that plodding round was junk mileage, but it ain't. They call it base training...establishing a base from which to develop your performance.
Are you on 20, 30, or more miles per week? Use your chosen figure as a target to get out when its dismal. I do.