Guy probably read something in an out dated exercise manual. As long as you take care of your body ie stretch, manage weight, eat enough, recover etc . . . the volume of running you do shouldn't matter. That said, blasting 150 miles a week at a 6 minute pace might leave you crippled at the age of 30, one of the reasons most kenyans don't have long careers. Train smart and listen to your body and you will probably be fine. 12 miles a day is serious volume - enough to be in really good marathon/half marathon shape. got out for 8 or nine this AM, but will double tonight.