Distance running requires run lots of miles. Like most runners, I like to link my fitness level to the number of miles I run everyday during the summer. During the winter I vary because I also do skiing as well. To being able to increase endurance and speed is to learn to swing your arms back and forth with your fists slightly clenched but not too hard. Your arms help drive your body forward faster. Thats where all the mileage comes in because endurance training helps build more fuel or in this case glycogen which can be stored in your muscles.
So just how many miles are enough? Well the answer mainly depends on a number of factors, primarily your genetically determined propensity to adapt more mileage. In other words just how much running can you physically and psychologically handle? In my experience I think its best to do as much as you can without losing hope of running faster than you want, getting sick, or injured.