(FORTUNE Magazine) - As the setting sun adds a golden spark to the ice in your tumbler, you can't help wishing you'd judged that eight-iron on 16 a little better - but there's always tomorrow. Soon you'll be sitting down to dinner with the spouse and planning your trip to the grandkids next Wednesday, or maybe it's Thursday, just before heading to the mountains for fishing and generally relaxing for a week or two, and then back to the old homestead for low-key taking it easy. Ah, retirement! The office is but a memory, and blessed leisure stretches as far as the eye can see.

Now let me ask you something: Is that what you really want?

Seriously. All that stuff is great, but a little goes a long way. The reality, which the financial-planning brochures and TV commercials rarely suggest, is that a great many working Americans don't really long for a leisure-filled retirement. Which is a good thing, because many of them won't be able to afford one. A recent study from Rutgers researchers concludes, "The vision of a traditional work-free retirement is yielding to a new notion of a work-filled retirement." A new survey from the Employee Benefit Research Institute is titled, a bit ominously, "Will More of Us Be Working Forever?"

We need to change retirement's image - and not just to include forever-young baby-boomers surfing and mountain-biking rather than playing shuffleboard. The real change is more radical: Growing numbers of us will spend at least part of retirement not in leisure at all, but working - sometimes for pay, out of necessity, and sometimes for free, just because it's so personally rewarding. It's a different picture from what we usually see, but it's fuller, richer, more complex, more real.

This FORTUNE Retirement Guide is all about today's retirement reality and how to prepare for it. Anne Fisher's inspiring look at second acts shows corporate retirees working hard at following their passions and loving it. For a surprising perspective on a key financial aspect of life in retirement, check out "How Much Will You Inherit?". If you want to go straight to the bottom line - retirement happiness - the singular Stanley Bing offers advice that we're pretty sure you haven't seen elsewhere.

Because today's workers need their investments to perform more than ever, we offer extensive help. The much anticipated FORTUNE 40 is the new edition of our recommended stock portfolio; last year's lineup once again beat the market. Yuval Rosenberg analyzes one of the sharpest anxieties of today's and tomorrow's retirees: What happens when nearly 80 million baby-boomers start retiring and selling their investments? We examine two legendary investors to whom workers and retirees have entrusted billions: fund manager Ken Heebner, who has performed extraordinarily well for 30 years but whose unpredictable style can be scary, and Mario Gabelli, who is suddenly accused of sundry bad behavior; Marcia Vickers looks at the contradictions of the man and his career.

Though today's workers are creating a new view of retirement, they're distressingly out of date on one of its most important elements: corporate pensions. I try to sort out the pension crisis. Because retirement is more complicated and confusing than ever, two of our experts, Ellen McGirt and Andy Serwer, answer a wide range of questions. And to see how a sample of real-life fortysomethings are thinking about retirement, read Andy's report from his Bowdoin College reunion.

If achieving a successful retirement seems impossibly demanding today, remember that the fundamental problem is one we're lucky to have: We're living longer. When you hear someone say 70 is the new 40, don't laugh. It's true. An American man born in 1850 had a life expectancy of 40; by 1990 the figure had risen to 78, and the numbers are higher for women. People didn't plan or save for retirement because for most of them, retirement never happened.

Today many people haven't faced how long their retirement could last. Even if they know average life expectancies, they usually don't grasp how far beyond the average they could live, surveys show. "Consider a couple age 65," says Mike Frazier, CEO of insurer Genworth Financial. "There's an 81% chance that one of them makes it to 85. There's a 25% chance that one of them makes it to 97."

All of which helps explain why only 13% of U.S. workers expect a fully leisure-filled retirement. It isn't easy to save so much money that you can forgo work completely for 20 or 30 years. But even if you could, would you want to? Among today's workers, besides those who expect to work for money in retirement, 27% say they plan to work part-time purely for interest or fun; 15% hope to start a business; 14% want to volunteer. Research supports the folk wisdom that activity and engagement keep you going; in one recent study people who worked longer outlived early retirees.

That's retirement reality in 2006. Challenging, yes, but the bigger picture is still a bright opportunity for an experience that previous generations didn't get to enjoy. Actually enjoying it requires you to behave in new ways, and especially to unhitch your mind from old habits as you think about what's ahead. With retirement, as with so much else, the future isn't what it used to be.