In his blog, Henry Fogel wrote recently about the time-honored tradition of conducting operas before before launching into a career as a symphonic conductor. Almost all conductors, including the famous ones, spent a good amount of time learning their craft in the pit of an opera house. Many conductors stay involved in the operatic world even after they have earned acclaim in the symphonic realm.
Fogel was the president of Chicago Symphony for many years and served as the CEO of the League of American Orchestras from 2003 to 2008. He also has an incredible database of classical music that you can search at HenrysRecords.org.