Imagine this nightmare: You're back in high school phys ed class, outdoors at the track, and it's seven a.m. You're shivering and half awake. An extremely fit jock, someone who normally wouldn't give you the time of day, measures your hips and upper arm flab, pinches your belly fat with evil-looking calipers, and orders you to run up and down the stadium bleachers. Next a trainer with a stopwatch demands that you do as many sit-ups as you can within a minute ("All the way up!") and as many push-ups ("Nose to the ground!") as you can before collapsing. Then one of the coaches times you running laps around and around the track with the rest of the class -- and you finish dead last.

That was not a bad dream. It was my first day of the six-week outdoor fitness program Boot Camp SF (for San Francisco). I hated every moment of that morning. Ever since I was a pudgy kid, perpetually picked last for the team, I had disliked running and doing sit-ups, dips, squats, lunges, and most other types of rigorous physical exercise. I was more fit than I'd been as a kid -- in fact, I was in pretty good shape -- but I still liked mellow, noncompetitive activities: a nice stretching class, a stroll with a friend, a bicycle ride. I preferred to glow rather than sweat.

But it wasn't enough. My doctor had told me in no uncertain terms that because I was in perimenopause, I needed to lift some weights and work up a good sweat regularly.

It turns out that all those miserable exercises the gym teachers foisted on us in high school are the very ones that can help keep us fit in our 40s and beyond. An occasional plank pose in yoga class won't suffice: We need regular strength training to build and maintain muscle and keep our bones strong; both of these things can help keep osteoporosis at bay. And raising our heart rate by running helps burn calories and keeps us in good cardiovascular health.

After my doctor delivered the bad news, I did what most well-intentioned people would do: I signed up for a gym membership. For several weeks, I donned my iPod headphones and joined a row of anonymous people jogging, Spinning, and stepping to nowhere in front of a bank of TVs. I pushed a few weights around. Then I became one of those people who expends a few calories feeling guilty about buying a yearlong membership to a fitness club and never going. To the list of things I hate, exercise-wise, I added being indoors working out on machines under fluorescent lights.

So when I saw an eye-catching ad exhorting me to "train outdoors" and realized the sessions were in Golden Gate Park, only a few blocks from my house, I signed right up. I thought it would be, well, a walk in the park.

It was not. But astonishingly, after that first grueling morning of tests, I found myself again setting my alarm for 6:15 a.m. I pulled on my shiny new kicks and went back for the second day. (One trainer told me that although I had never actually run in my running shoes, they had lost their spring from the five years of walking around I'd done in them. So I had bought a new pair.) The trainers had divided the 30 of us into three groups, based on speed. No surprise, they put me in the "slow" group (I prefer not to call it "remedial") with mainly trim women who seemed pretty darned fast to me. I was happy I wasn't in the gang of marathoners with the Scottish coach who had trained with British special forces and is an expert in jujitsu and boxing. Still, our trainer, Tracy Hicks -- a blue-eyed 38-year-old with so little body fat I wondered how she managed to have dimples -- didn't cut us any slack. She ordered us to run hills and do countless push-ups and crunches in the grass. As we gasped for breath, she would remark cheerfully about how the glorious morning sun was streaming through the redwood trees.

After an hour, I was surprised to find that I had done everything Tracy had asked, more or less, and had survived. I dragged myself back home, and there in the shower, at 8:15, it occurred to me that if I did nothing worthwhile the rest of the day, I'd already been extremely virtuous. The virtuousness spilled over to breakfast. Cream in the coffee? No, make it nonfat soy! Steel-cut oats was the obvious choice, as was the healthy salad I packed for lunch. At dinner I realized there was no way a second glass of wine was going to help me run sprints in the morning. Already I was on a roll -- even if I was extremely sore.

Boot Camp SF lasted an hour a day, four mornings a week, with an optional Saturday session (Okay, I never went). Each day was a different workout. Mondays we might run trails to the lake. Tuesdays we jogged the hills behind the dahlia garden and did crunches. Wednesdays mercifully, we had off. Thursdays we worked our legs -- lunges and squats near the Conservatory of Flowers -- and Fridays we concentrated on arms, doing dips and push-ups. For all the fancy fitness equipment available in the world, we got in a whole-body workout with nothing more than the park's benches, trails, and grass, along with some resistance bands for arm exercises.

Prior to boot camp, I had rarely run farther than it takes to catch a bus. On the intake form, which asked if we could run three miles, I wrote, "with a gun to my head." But we started out slowly, with breaks to do a few stretches and arm exercises while we caught our breath, and then increased how long we ran until, by the third week, we were running the entire hour. I had never in my life run for an entire hour. I was slow, but not always behind; Tracy had a neat way of having the fast runners circle back so I wouldn't get that defeated feeling of always being last. She also taught us tricks to make running easier, such as leaning into it and keeping our limbs loose. By the end of the six-week session, I was voluntarily running the half mile from my house to our meeting place in the park.

Golden Gate Park is outdoors, so outdoor things happened. Hawks circled overhead, bunches of purple flowers bloomed, rosy morning clouds floated over the treetops. Occasionally, it rained. I assumed that rain would cancel class, and we'd all drive to a cafe for lattes and frosted orange currant scones. But no: "It's fun in the rain," Tracy said. One day we did sprints in a torrent of pelting drops, then took shelter under a building by the playground to perform six kinds of abdominal crunches. By the time I got home, I was soaked to the skin -- and strangely exhilarated. A shower had never felt so good.

After three weeks of religiously attending the seven a.m. session, I accidentally slept in and showed up at Tracy's 8:30 class. There, instead of the 25- to 35-year-olds who were training for marathons or trying to fit into bridal gowns, I was delighted to find women in their 40s -- including a few who ran as slow as I. My people! Since Karen Judge, 46, a jewelry designer, and Deborah Wade, 45, an actress, ran at my pace, we could actually talk to one another. I stayed in the later group. It was great to have workout pals to complain and laugh with. Both Karen and Deborah had been to boot camp before. Even in Lycra workout pants, they looked pretty great -- though they assured me that wasn't the shape they were in when they started.

I also started to see some changes in me. Friends commented that my body was looking "tight," and I noticed muscles nudging through my squishy topography. I took a ski lesson with an instructor who was surprised that I wasn't out of breath my first day at altitude, and that my quads weren't squealing after skiing 15,000 vertical feet. And I had a date with a doctor who, after I led him on a walk up some of San Francisco's steepest hills, pronounced me fitter than 99.9 percent of women my age. On our last day of the session, the coaches tested us again. "Fitter than 99.9 percent of women my age" turned out to be an exaggeration -- except, according to the American College of Sports Medicine standards, in the sit-ups category, where my 64 per minute hit that mark. I went from being able to do 14 pushups at a time to doing 23. I ran a mile and a half in 15:18, versus 17:50 when we started. I had lost six pounds, plus an inch and a half around my waist and an inch of upper arm flab. I had shaved off a couple of percentage points of body fat. (My booty hadn't budged, but there's always hope.)

Running around the track, I was still dead last, even in the later group, but I had left my old self in the dust. I've even signed up for another session of boot camp, and I mean to kick some ass -- even if it's just my own.

Train with the Best of Them

Fitness boot camps have been springing up around the country, attracting people who prefer a challenging outdoor workout with a trainer and a group to going to a gym. Some of these boot camps take the military metaphor a bit far, with Web pages that feature guys with crew cuts and camouflage uniforms barking orders to the tracksuited troops, but others take a more casual approach, simply using the great outdoors to help their recruits get into great shape. Many boot campers use the typical four- to six-week session to overcome a fitness plateau or accomplish a goal (run a race, lose the love handles); others return session after session because they've found a workout they can stick to without getting bored. To find a program near you, search online for "fitness boot camps." Chemistry with the instructor is important, so ask to try a class or at least have a talk with her before you commit. If you're motivated by a positive, supportive voice cheering you on to one more sprint and the teacher's style is closer to Patton at the Battle of the Bulge (or vice versa), it won't be a good fit.

More from Your Over-40 Health Guide

Originally published in MORE magazine, May 2008 as "Busting Out My Inner Jock."

MORE articles in this Topic