My back hurt all day yesterday but I still set my alarm to run this morning. At about 3:30 by youngest daughter that was throwing up last night said "Daddy, Kitchen" which being interpreted is "feed me now". My back was still hurting as I was feeding her so I adjusted my alarm and slept in. I'm guessing my core muscles are just a little to week for the 40+ miles a week. I'll have to incorporate a few new excersizes.