My water aerobics instructor is 38 years younger and 80 pounds lighter than me. I think that is why, 45 minutes into class, I am praying for early dismissal while she could do another hour easy.

I do not like exercising. Water aerobics is my compromise. I do it because I must. There is a rule for seniors: “Use it or lose it.” This applies to everything but weight.

For a few years after retiring, I walked two miles a day. Then the knee pain became so severe I went to see the family doctor. He suggested daily walks on hard pavement weren’t helping my knees. I was “bone on bone,” he pointed out and a prime candidate for knee replacement.

I took Vioxx for the pain and replaced walking with swim aerobics. My goal was to keep good muscle tone in my legs for the time when I did surgery.

That time came four years later when I could no longer spend 30 minutes walking the cement floors of Costco to shop. In truth, I could barely do 10 minutes. I was one of those folks hunched over the grocery cart bracing myself with my lower arms for extra support.

Another round of tests and X-rays revealed both knee joints needed replacement.

I had the first surgery six weeks later. Out of surgery at 8 p.m. and up the next morning at 8 a.m. for physical therapy. The pain of standing on a new knee is not for wussies. Neither are the therapy sessions. These exercises are hard work that must be done.

It took a few weeks and a judicious use of pain pills to move past the worst pain. “Judicious” is the key word here. Take a lot of those suckers and another body part, somewhat immobilized by the painkillers, can refuse to move regularly. This is pain you do not want at any cost unless you just love enemas.

Two weeks after surgery I was back in swimming pool as well as doing three one-hour sessions a week with a physical therapist. I moved from walker to cane in two weeks. At four weeks the therapist said I could get rid of the cane.

My surgeon, assuming a 64-year-old overweight woman like me would still be hobbling in on a walker at her six-week checkup, was surprised.

“We should have videotaped you going through therapy,” he said.

Two years later he replaced the other knee. Surgery and recovery went without a hitch. Exercise in the water was a key element then and still is. At first, I just walked in the water for 45 minutes. Then I added exercises recommended by the physical therapist.

The new knees can do a lengthy shopping excursion and chase grandchildren. They feel natural and work pretty much like the originals. The surgeon did advise against mountain climbing and tennis. No problem; I hate exercise.

The knees have certain limitations. I can’t get on my hands and knees to scrub and wax floors or other chores that require kneeling. Oh darn. Falling down is not recommended, either. I am careful not to take risks and to watch where I’m walking.

During the summer, when the community’s outdoor pool offered swim aerobics, I replaced my personal water routine with group “therapy” four nights a week. Now the same program is offered at an indoor pool.

More than half the class is under 40, but there are also a few women my age and older doing the drills.

We keep up.

Genie Faulkner, 73, does two hours of line dancing in the morning, plays cards, gardens and then shows up for swim aerobics at 5 p.m.

“Use it or lose it,” she chirps as she adds extra arm movements and more splashing to the jumping jacks or passes me as we jog backward.

I hear that message clearly. Splash on, girlfriend.

Linda Bryant Smith writes about life as a senior citizen and the issues that concern, annoy and often irritate the heck out of her now that she lives in a world where nothing is ever truly fixed but her income. You can send e-mail to ljbryantsmith@ yahoo.com.