We all look with a little bit of envy at those running in the big one this Sunday — the marathon. But many of us have a secret. The marathon has competition — the triathlon. This race of swim, bike, and run is becoming more and more popular. Even celebrities have started to pick up the sport.

The triathlon has gained traction in marathon city New York. The national triathlon organization, USA Triathlon, has more than 80,000 registered members. New York has more members than all but three other states. It ranks fifth in the number of triathlon clubs.

Maybe this is because the city is a good habitat for the triathlon — sites for the different sports are close to one another. Central Park is an ideal place to train for the run and bike. After working out there, you can head over to Asphalt Green, which is by the East River between 91st and 92nd streets, and jump in the swimming pool. Asphalt Green has both an indoor and outdoor pool, great for training during the warm months, and even has a triathlon club and triathlon training programs.

I’m writing all this because I’ve had my own love affair with a triathlon. I used to run, marathon style, back in those college days when there was time to jog 13 miles a day. A good course was from the campus of the University of Pennsylvania all the way down to the river and then back.

Why? Running is a great workout. Moreover, it becomes addictive. No other kind of exercise ever feels as good — at least, at that moment. What can beat the runner’s high? Those of you who have ever run more than eight or so miles without stopping know what I am talking about. It is probably similar to the high anorexics get from not eating, a kind of prolonged euphoria. An added bonus of running is that it gives the body a lean, sleek shape, which is hard not to like.

But, ultimately, running takes a toll. And after experiencing injuries produced by or exaggerated by running, one begins to wonder how beneficial it really is. In my case, chondromalacia, when the knee cap slides off track, claimed my right knee. Tendinitis took over both knees. All of which made me rethink all this running.

And then I found out about triathlons. Originally, triathlons were developed as a means to train for a marathon. Now, people find triathlons to be a much more healthy and holistic form of competition. Working with a bunch of other people on different sports sounded intriguing — fun, even. When was the last time running 20 miles a week even sounded like fun?

In his column about the death of four-time Olympic gold medalist in the discus throw, Al Oerter, Sun columnist R. Emmett Tyrrell laments the loss of the “Olympic ideal of amateurism.” He writes, “With its passing has gone the love of sport for its own sake, the sheer fun of competition.” Triathlons, though, are a resurgence of exactly that — fun, balanced competition.

I wasn’t sure I could do a whole triathlon — a 1.5k swim, 40k bike, and 10k run — for my first one. But there was a tri option for me — one that was in between a standard length and a sprint length. Sprint tris are about half the distance of regular length ones. Nowadays, triathlons come in various distances.

My tri debut was in the Northern Columbia triathlon this summer in Columbia County, N.Y. The swim was a quarter mile in a lake. Yes, there were weeds. The bike was 19.8 miles, and the run, a good portion of which was uphill, was 4.5 miles. In terms of this Sunday’s Marathon, that is equivalent to running over the Verrazano Bridge and through Sunset Park.

Next I tried out yet another form of competition — a biathlon which consists of a run, bike, run (2 miles, 12 miles, 2 miles) in Central Park. In some ways it was harder than the triathlon I did. Just by starting with a run instead of a swim makes the bike harder. But my knees barely hurt.

Biking alone or swimming alone does not provide a complete workout for me, but combining them with a run, one that is not too long, works perfectly.

A free-for-all charge into refreshingly cool summer water is the most exciting way to start a race. Biking along scenic country roads helps dull the pain especially when trying to ride up hills, and the run allows you to take it all in.

The greatest challenge: The transition to the run from the bike is incredibly hard on the legs. In fact, that moment when you start stretching out your legs after riding is the most painful part of the race.

Triathlons may also be growing in popularity because they fit what might be called our national culture of ADD. They are perfect for people with short attention spans. By the time you get bored with one exercise, you are already transitioning to another.

And the sport certainly seems to supply a sense of normalcy to everyday craziness in New York. With most things taken to the extremes nowadays, it’s nice to find something that doesn’t overtake your life, but rather fits into it. Triathlons provide exactly that. They modify the extremes of marathons, or bike-a-thons, or long-distance swimming.

Did I mention the shopping? You not only get to shop for running shoes and an outfit, but bikes and the biking gear, wetsuits, tri suits, goggles, the list can go on forever. Some triathletes will spend thousands and thousands of dollars to make sure they have the newest, top of the line, dri-fit, skin-tight, super fabulous looking gear. I didn’t go that far, but I did get tri shorts, a tri top (it has a ventilation system and built-in sports bra), and swim goggles.

There is even an entire store, SBR (Swim, Bike, Run), at 58th Street and 7th Avenue, that is devoted to triathlon attire, has its on tri club, and acts as a community center for triathletes. Triathletes wear the same outfit for the swim, bike, and run parts since that significantly cuts down transition time between events.

After feeling my own tri top begin to ride up during the run and seeing fellow triathletes lose seconds over stumbling into and out of their clip-on bike shoes (I don’t have them), I can pass on an important triathlon rule: Whatever you purchase, just make sure you get your gear figured out far in advance of the race.

As for your bike, there is a great bike store in Park Slope, R&A Cycles, that offers a wealth of information not just for bikes, but also for specific triathlon bikes known as time trials. Instead of leaning your hands on the handle bars of a road bike, your forearms rest on a flat, vertical version of a handlebar, which is supposed to help increase your speed, but also takes away from your control of the bike.

For those in Manhattan, Cadence Cycling in TriBeCa has triathlon coaches and training programs.

What else? Training is easy to fit into a busy New Yorker’s schedule. And tris are also quite easy to train for. There are an overwhelming amount of tri clubs in New York City alone and fellow triathletes are more than eager to share their advice on training or train with you.

For those who grew up playing sports, whether it was because the school you went to required it, or because you simply like being active, triathlons are a great way to feel like you are playing a sport again. And they fill the desire for sports competition.

So as they celebrate the November marathon, New Yorkers might like to recall that there is another competition to think about. The 9th annual Nautica New York City Triathlon will be held over the summer. This year it had almost 3,000 competitors — 2,998 to be exact.

My advice? Start workouts now. The idea is tantalizing. After all, you will be finishing your Sunday post-practice brunch while marathon man is still pounding the pavement.

kherrup@nysun.com