“You must do the thing you cannot do,” Eleanor Roosevelt once wrote. It’s the only way to overcome the fears we all face in doing something new, she thought, and take a leap into the unknown.

Kate Williams quoted Roosevelt earlier this week here when she accepted a $25,000 Purpose Prize, one of the awards given annually by Encore.org, a San Francisco-based nonprofit that works to engage baby boomers in "encore careers" with a social impact. The awards, now in their ninth year, recognize trailblazers over age 60 who have tackled social problems creatively and effectively. Cash prizes range from $25,000 to $100,000.

Williams, 72, lost her eyesight to a rare degenerative disease after a long career as a corporate human resources professional. She overcame her own fears, first by moving away from friends and family in Southern California to start over in San Francisco and later by starting an employment training program for the blind. Today, she runs a similar, larger program for the national non-profit organization Lighthouse for the Blind.

Encore.org’s mission is to promote a game-changing idea: Greater longevity and the graying of America present opportunities, not problems. This year’s Purpose Prize winners underscore that point. They’re rock stars in the world of social entrepreneurship, having started organizations that work on issues like sex trafficking, disaster relief, autism and education in impoverished neighborhoods.

The idea of second careers with social purpose has broad appeal. Millions of older Americans want to stay engaged and work longer, sometimes out of economic need but often out of a deep motivation to give back. An Encore.org survey this year found that 55 percent of Americans view their later years as a time to use their experience and skills to make a difference, though just 28 percent say they are ready to make it happen.

Many people have trouble figuring out where to start - which brings us back to Roosevelt. Fear of the unknown is a key hurdle in starting down a new path later in life, and I had the chance to ask some of the encore experts gathered for the awards about how they would advise others seeking to begin.

The juices get flowing when people connect their experiences and knowledge with a problem they are passionate about. But first they have to make the leap.

“I had been in the corporate world, not part of the blind community,” Williams says. “I was frightened, but what I thought would be overwhelming turned out to be a beautiful thing. As soon as we started our training classes, I was hooked.”

The Lighthouse for the Blind program has worked with 100 blind job seekers over the past three years, and has placed 40 percent of them.

David Campbell, winner of a $100,000 prize this year, wanted to help after the Indian Ocean tsunami that devastated parts of Southeast Asia in 2004. A senior executive at several software and Internet technology companies, he figured he could help by creating a Web-based tool to organize volunteer tsunami relief efforts. That led him to start All Hands Volunteers, which has worked on 45 disaster relief projects in six countries and dozens of U.S. locations. The non-profit uses the Internet to route volunteers to places where they can be put to work effectively.

“People just want to know that if they go, they’ll have a place to sleep that won’t be a burden to the local people, and a contact to start with," he says. "We give you exact instructions on how to get there, and assure that you’ll have a bunk bed, food and someone will have organized work and that you’ll have the right tools to be productive.”

Campbell talks often with people looking to get started on encores. “I always advise people to start by volunteering with some organization with social purpose - it’s an easy, great way to start. But the question many people have is, ‘Which one, and what might I do?’ ”

Campbell suggests people consider geography and the focus of the work. “Do you want to work locally, nationally or internationally? Do you care about health, education or some other thing? That starts the conversation and helps people narrow it down.”

Then, he says, visit a non-profit that interests you, and take the time to understand its needs.

“Be willing to help understand the mission, and do whatever it is they need help with. And don’t treat volunteering as a casual activity. You need to commit to a certain number of hours of work a week as though it were a paying job, and take responsibility for it.”

To paraphrase another famous Roosevelt, the only thing you have to fear is fear itself.