Joel Friel began advising athletes from behind the counter of his running store in the 1980s. Within a few years, he was coaching triathletes, helping to create a new niche: personal coach. By the time he retired last year, Friel, author of “The Triathlete’s Training Bible” and of various Web sites, had developed an avid following among generations of elite runners, bikers and triathletes.

What has he learned after 30 years of studying competition and high-level performance? Athletes shouldn’t slow down as they age; keeping up the pace gives them the best chance of staving off decline.

Friel, 70 years old, uses himself as an example. “I’ve been watching [for any noticeable decline] all these years,” he says, “and quite honestly, my performance has remained much the same.”

Every day, Friel gets on his bike for at least two hours, often adding a game of golf or a session of weightlifting. He lives part of the year in Boulder, Colo. — until the temperatures begin to dip — and part in Scottsdale, Ariz. — until the heat is too much for him to take. Although he gave up triathlons seven years ago, he still races bikes. While he was on vacation in Tuscany recently, he stopped pedaling to talk to The Post.

Joel Friel, a retired personal coach, says he has been watching for any decline in his physical fitness, but “quite honestly, my performance has remained much the same.” (Tina Celle Photography)

Let’s talk about your physical fitness. You’re now 70 years old.

Back in the late 1970s, I was running a little bit, but not too seriously. Then in 1979, I tried a marathon. Then the next you knew it, I was hooked on marathons. In 1984, triathlon became much more important to me. I tried a few of those and fell in love with that sport.

In 2006, when I started having knee trouble, bone on bone in one of my knees and cartilage around the edges, I realized in talking to people in the medical field that I really had two options: I could quit running or I could have surgery to do some repair of the cartilage.

I decided to stop running, which meant giving up triathlons. I started racing bikes, though, and I’ve been racing bikes every since then.

What have you noticed about aging, both in yourself and in your athletes?

I began to think about this in my 50s. I started reading the research on aging because I thought I’d start seeing a decline and I wanted to know what it meant.

What stood out for me: Performance would slow down in your 50s; there’d be a noticeable decline. But the most noticeable decline would occur someplace around age 70. Until that time, it would be such a small decline, it would be hard to measure.

For me, I’ve not seen a big drop-off yet, although this year for the first time something did begin to concern me. There’s this thing we call in cycling your FTP (functional threshold power), which is a measure of how much power you can put out for an hour. It’s kind of a standard that everything is based on. For the first time in all those years, I had a hard time getting the numbers up.

What did you do?

There is a growing body of evidence that high-intensity training is one way of maintaining aerobic capacity, which seems to be the thing that drops the fastest in aging athletes beyond 50s, 60s and 70s. [High-intensity training] means you are doing things well above the FTP, which are very hard workouts and intervals. I realized I had done what every aging athlete I know of does in my 60s: fewer and fewer interval-type training workouts and more steady-state type workouts and voracious workouts.

So I began to do high-intensity workouts, which I hadn’t done for a year or so. And, sure enough, my numbers have gone up again.

So you’re feel like you’re defying aging?

I am writing a book on that, on aging athletes and what I’ve learned about trying to maintain or improve performance as we get older, knowing that it’s not going to be maintainable until you’re 80-, 90-some years old. There is going to be a drop-off, but it can be slowed; the research supports that.

What’s your day like?

I get up fairly early, by 5 at the latest, typically by 4.

You should wake up when you wake up. One thing I’ve discovered as I’ve gotten older is that I don’t need as much sleep. I typically go to bed by 9 o’clock. I get about roughly seven hours of sleep, sometimes more, sometimes a little less. I just wake up, and when I’m awake, I get up. I believe if you are using an alarm, you’re not getting enough sleep. That is what I tell the athletes I work with: You should wake up when you wake up.

What do you eat to keep yourself in good shape?

In 1995, I changed my diet. I had been eating a very-high-carbohydrate diet, lots of starches, especially breads, cereals, pastas. A friend of mine, a runner, kept telling me about a project he was working on, which he was calling the Paleo diet, short for Paleolithic. He kept telling me about it and the research on it.

He finally said to me, “Why don’t you try what I am suggesting for one month? If it doesn’t work, we won’t talk about it anymore.” I started eating that way, and for about two weeks I felt miserable. I wasn’t recovering well. I was hungry all the time.

But after three weeks, I felt really good. My training started to come around really nicely. I went for four weeks, then five weeks. I was doing a triathlon at that time, and I would normally begin to experience upper-respiratory conditions: a sniffly nose, even a sore throat, which is quite common if I got my volume [of exercise] very high, depending on how many hours I trained in a week. I felt fine. I didn’t have any sore throats or head colds. I wasn’t breaking down. I realized something was happening: My diet was changing my physiology.

I’ve been eating that way every since. That means lots of protein in my diet, lots of vegetables.

So just vegetables and protein?

Last year, Tim Noakes at the University of Cape Town, South Africa [a proponent of high-carb diets for athletes], an MD, started tweeting about changing his diet to eating low carbs, high fat. He was losing weight. His running times were getting better. He’s been a runner for decades, as I had been. I thought I would try it.

I started to cut back on fruits and starch in my diet. I lost a bunch of weight also. I was never what you would call overweight. But in the last few years, I experienced what all aging athletes experience, which is I was gaining weight in the off-season.

I’d go from 154 pounds to 165 pounds over the course of winter, and in the spring I would try to take it off by restricting calories. I changed my diet and noticed I began to lose weight immediately. In the course of eight weeks, I lost 12 pounds.

I was at race weight, which had always been a little bit of struggle every spring. I thought maybe I [would] lose some of my performance. It wasn’t a problem whatsoever.

When you say “high fat,” what do you mean?

Lots of fats. Bacon. Butter. Cheese. Really anything that is high-fat, saturated, not a trans fat. I eat a really high-fat diet. Basically the only carbohydrate I get is fruit at breakfast: half of a piece of fruit, half a pear, or sometime during the day I might have another piece of fruit after a ride.

So no sugar, cookies, alcohol?

My wife and I have a glass of wine every evening before supper. We don’t eat cookies, ice cream. We don’t keep stuff like that in the house. We just don’t eat it.

People think it’s a strict diet, but I enjoy what I eat. I am not depriving myself of anything. The research [I read] doesn’t support that fat causes heart disease or that cholesterol is the cause of heart disease.

Let’s talk a bit more about how you train as an older athlete.

I will do different types of workout. The most basic is called steady state thresholds. I’ll do that workout a couple times a week. It’s long intervals with short recoveries. It will be on hills or flats.

I’ll also do what I call an aerobic threshold workout several times a week, which is just a ride in a steady state for a long time, like two hours.

And I do intervals, things like three-minute repeats on a hill, going very hard up hill, high intensity. Then I recover for three minutes. I’ll do that maybe five times. Makes for a hard workout.

Plus I do recovery rides. Just go out and smell the flowers.

Also, I lift weights several times a week. Have a weight room at my home.

Do you train a young athlete differently from an older one?

In some cases, yes. Young athletes can make lots of mistakes in terms of nutrition and sleep and get away with it. Your body is much more forgiving when you are young — a little bit of sleep and you’re ready to go. You can punish it, and it will bounce back because you have hormones coming out of your ears.

After about age 35, hormones drop off rather significantly. At my age now, hormones are few and far between, I’m afraid. I have to be very aware of my body all the time. I make sure I rest enough. I stay off my legs, [and do other] things that are going to help me recover.

As an older athlete, what do you have over younger athletes?

The most important thing is I understand my body. I know what I can do and what I can’t do. I just understand how to train.

I’m highly motivated. I always enjoy going for ride, but sometimes younger athletes, although they want to, they can’t. They have so many things in their lives that interfere [such as] trying to start a career and raise a family.

I’ve been through that. Now I can do things as I want to do them. That’s one of the beauties of getting older: You have much more freedom in your life. I’m taking advantage of that in every way that I can.

Hambleton is a freelance writer and documentary filmmaker.