While people can often estimate how much they might need for 10 or 15 years of retirement, that calculation becomes more difficult for retirement that could last 20 years or more. Gary Waters/Ikon Images/Getty Images hide caption

itoggle caption Gary Waters/Ikon Images/Getty Images

Gary Morse, with wife Sharon, in 1999. Morse transformed a mobile home park in Florida into The Villages, a retirement community of more than 100,000 residents. Stephen M. Dowell/Orlando Sentinel hide caption

itoggle caption Stephen M. Dowell/Orlando Sentinel

Manuel "Manny" Aguirre, 80, has been mixing cocktails at Musso and Frank Grill in Los Angeles for more than two decades. He works part time and could retire — but he doesn't want to. Courtesy of Musso and Frank Grill hide caption

itoggle caption Courtesy of Musso and Frank Grill