The idea of high mileage for all runners that use their aerobic system as a primary energy source for their race distance is an idea made famous by Arthur Lydiard. His ideas of long slow distance and training are commonly misinterpreted. One of his most famous athletes, Peter Snell, was running 20+ mile long runs before his double gold performance in the 800m and 1500m at the 1964 Olympics in Tokyo. Why would someone run 20 miles when their event is less than a mile? To maximize your aerobic sytem one must continually stress it to continue to improve. Everyone running 800m and up should have a weekly long run as part of their training.
On the other hand, runners can become WAY to obsessed about mileage. Ryan Hall wrote in a recent post, that he used to go out for a 2 mile run before the week was out just to hit his magic number of at least 100mpw. He goes on to write that now in his running career exact mileage has become less important. What he finds most beneficial now is "hard, quality workouts followed by adequate recovery and even making sure to over-recover." That doesn't mean that Ryan is running less weekly mileage. It just means that he knows that the recovery is most important, and that those 2 extra miles my hinder his trainng more than enhance it.
It is still important to keep track of your mileage. You can tell when you have done more than you are used to and may need a recovery week or an extra recovery day. My advice is schedule your "workouts" first, meaning long runs, tempo runs, and intervals. Then fill in with other easy runs that will challenge your aerobic system and facilitate your recovery not impede it. Also, continue to challenge your aerobic system by doing a "little bit more than last time." For example if you were running 30mpw without trouble, then increase to 35mpw during the same stage in your training.
Have you ever wondered what a 140 mile week looks like? Nick Arciniaga recently posted on a typical 140 mile training week during his marathon training. Check it out. At first 140 miles sounds insane right? And it is, but when you think about how Nick trains it is not as much as you think. He has it split up into 13 workouts over 7 days. Nick averages some where between 9 and 10mph on even a long run or recovery run, sometimes faster. If he averaged 10mph over the whole week that would be 14 hours of running a week. That is 2 hours a day. If someone who averages 6mph on their training runs ran 2 hours a day they would run 84 miles that week. That would be a lot of running for them too but it would be an equal training stimulus for them (140mph is to Nick as 84mpw is to 10min/mi runner). It would take the 10min/mi runner 23 hours/wk to run 140miles!!!! Now that is insane!
Nick can run 100mpw in about 10 hours of training. It would take me about 13 hours/wk. Even at top marathon shape I am lucky to get in 5-6 hours/wk.
So Nick runs 2 hours a day. What does he do the rest of the day? All the little things to make sure he is recovered enough for the next workout. Must be nice.