Massachusetts Secretary of State William Galvin has plenty of practice as gubernatorial understudy, a role he'll reprise when he becomes lieutenant governor in a week. The 62-year-old Brighton Democrat takes over the post when Lt. Gov. Timothy Murray resigns on June 2. As Lieutenant Governor, Galvin will serve as acting governor when Gov. Deval Patrick travels out of state. And he could serve in that role through the remainder of Patrick's term if the governor takes a post in the Obama administration.