Six years ago, as I was just getting started running, I sent a fan letter to experienced ultramarathoner Paul Ammann thanking him for his online race reports  and sketching out the naïve training plans I hoped to pursue in ramping up my mileage and improving my speed and endurance. Paul gently replied with superb advice — advice which came to mind again recently when a new friend started training hard, got injured, and had to scale back expectations. So it's not lost again, Paul's comments:
IMHO, this is a very aggressive schedule, by which I mean that the risk of injury strikes me as quite high. Your body can handle all kinds of ridiculous stresses - but only if you give it plenty of time to adapt. Many people - including me, I admit - are impatient with running goals. And many people - including me again - are often sorry about that. I've read a lot of race reports that essentially read:
"I ramped up fast and then I visited my doctor"
I have a more or less continuous "injury recovery" program in place. My ankle, or my knee, or my hamstring (all on the left leg, curiously) is typically in need of some form of maintenance.
Patience really is a virtue, and the reward for patience is very concrete, namely injury avoidance.
If I were doing my first marathon over again, I would probably plan to walk a third of it (mostly in the beginning). It would be much more pleasant that way, and the risk of injury would be near zero. Ultra marathons have taught me that extensive, deliberate walking is not only "ok"; it is much faster than running til you can't.
Look at it this way - you aren't out to run one marathon. You are out to run so many that you lose track of them. Make one of those future marathons the qualifier.
btw - 20 miles a week is plenty to run a marathon. Don't let the logbook run your life. I believe a fair number of people run 100s on just 30-35 a week (me included). Again, injury avoidance is the main goal. I would recommend crosstraining over increased mileage.
And listen to *your* body. It will definitely be talking to you!
(quoted with permission) - ^z - 2008-06-02