If there were an award for longevity at a single agency, Hill Holliday’s new CEO probably would win it. Karen Kaplan, named today to succeed Mike Sheehan as the top executive at the agency, joined Hill Holliday as a receptionist in 1982. Back then, the shop had 125 employees at a single office in Boston; today, the agency employs 950 staffers at offices in Boston, New York and Greenville, S.C. Total revenue, from accounts such as Dunkin’ Donuts, Bank of America and Liberty Mutual, is estimated at about $200 million. Kaplan, 53, takes the mantle nearly six years after she became U.S. president and as Hill Holliday prepares for a big presentation. The agency is partnering with fellow Interpublic Group shops Campbell Ewald and Lowe to pitch Cadillac’s creative account later this week. Kaplan took time out from pitch preparation to discuss her influences and why so few women run creative agencies these days.