Yesterday, for the first time, I really started working on thumb position. Actually I was doing the exercise recommended by Rufus Reid in the latest BP, he says you should do major scales all the way up and down the fingerboard; I went up to the first 3 notes in thumb position. My thumb hurts like hell! Also it's pretty hard for me to get the D and A strings...any pointers? How many hours before the blisters develop into callouses?