You're retiring soon and all you can think about are all those free afternoons on the golf course. Or loafing all day without anyone telling you that report's due tomorrow.

Well, better think again, advises Grace W. Weinstein, who has just written a book on retirement planning. She points out that you may live 20 years or more - a full quarter of your life - after retirement, and that's a lot of golfing and loafing for anyone.

One bored but healthy 18-year-old man told her, "If I'd expected to live this long, I would have figured on doing something besides play golf."

Many people look forward to retiring. All of that time to do what we want, when we want. Others dread the prospect.

Studies indicate that the way one looks at retirement is usually what one finds, Weinstein says. Contrary to many popular myths that locking up your office desk for good signals decrepitude and death, she says, retirement should be anticipated positively as "a new and potentially exciting stage of life."

After all, she says, "Old age doesn't start until 80," and maybe not even then for some. Often one's health improves once the stresses of the job are gone.

"Both individual experiences and sociological studies indicate that the vast majority of men and women, doom-and-gloom reports notwithstanding, like retirement and get used to it quickly," she writes in "Life Plans: Looking Forward to Retirement" (Holt, Rinehart, Winston; 252 pages, $4.95 paperback).

Weinstein, whose home is in Teaneck, N.J., is 43 and a long way from retiring. A lecturer and consultant on children, money and personal topics, she writes a monthly column on retirement for "The Elks Magazine" and begins a column, "Your Money," in August for "Good Housekeeping."

Weinsteiin, who was in Washington the other day, strongly advocates planning for your retirement well before you leave the job. You may think there are enough jobs around the house to keep you busy forever, but you'll probably zip right through them in a few weeks and be looking for something else to do. She offers these tips:

Rehearse your retirement. Practice living on your retirement pay, which in most cases will be less than you are making on the job, and invest the difference as a retirement nest egg.

Develop and practice your expected retirement activities, whether it's volunteer work, continuing your education, utilizing creative talents or starting your own small business. "People who take the time to develop outside-of-work interests while they are still working are less likely to suffer from disenchantment," she writes.

Talk things over with your spouse. One of you may dream about a year-round life in that Vermont cottage you were able to buy for a summer getaway. Great in August, but a winter in the snowy woods may be the last thing the other half of your family wants.

Take great care in deciding where you are going to live. One reads often about the crowds of retirees flocking to Florida, but most people stay home with family, friends and roots. Remember, if you move to a new city to be with a son or daughter, that child's job might force him or her to move away from you.

Try tapering off the job with a reduced work week, if you can manage it. Often writers, lawyers, doctors and other professional people can ease into retirement a few hourts at a time. Whether at 55 or 75, it's up to them.

Weinstein says she has met many happy retirees. And those who are the most content are doing something they like that gives them the feeling they are needed and are making a contribution. Also important is maintaining a continuing relationship with people. Take time before you leave your job to strengthen your relationships with family and friends, she advises.