Joan Macdonald has not always looked like a bodybuilder. At 71, she weighed 90kg (14st 4lb), and had rising blood pressure and kidney troubles. She was also on medication for cholesterol and acid reflux, and her doctor wanted to double the dose.

Her daughter, Michelle, expressed Macdonald’s dilemma bluntly. “You’re going to end up like your mother did in a nursing home!” she told Macdonald. “And people are going to have to look after you. Do you want that?”

“Of course I didn’t want it,” Macdonald says now. “I was sick and tired of being sick and tired.”

Macdonald’s thoughts swirled for two weeks. She thought: “I want to earn Michelle’s respect. I mean, she loves me, but you can love a person without even liking them.”

She left her home in Ontario, Canada, to join Michelle and her husband, both fitness coaches, in Tulum, Mexico. Macdonald learned to make protein shakes. She visited the gym. She followed Michelle’s workout programme, using the machines, then light weights – 2.5lb (1kg) – working up to heavier ones. She mimes raising a barbell and lowering it behind her head. She can do this with 25lb (11kg).

“Wow! Your back!” people in the gym told her. “It’s so defined!” They took photos. “I’m going: ‘Wooh! That looks pretty good! I’ve got some muscles here.’” Within nine months, she was off all medication.

“There is a misconception that people over 65 cannot produce hypertrophy [growth] of the muscle,” says Mark Peterson, an associate professor of physical medicine and rehabilitation at the University of Michigan. “Muscle is a tissue that can adapt with stimulus.” The key is to check with a doctor first, and start slowly.

On Instagram, Macdonald posts photos of herself in sports tops, or jumping waves in beachwear. She has 1.4m followers and a partnership with the retailer Women’s Best. “I have a tendency to do things that anyone else would say: ‘Oh my God! What’s she doing now?’”

Macdonald celebrated her 75th birthday this year by zip lining. She has always liked a challenge, always liked “building something”. In DIY projects, her husband is supervisor and Macdonald “dogsbody”. But now, she says: “I’m building me instead.”

Macdonald has lived a lot of her adult life in a medicated body with a variable weight. So why was she able to make the change at 71? “I think I’d met a new low in my life. I’d maybe touched bottom.

“I was busy with my family. You forget about yourself. I wish I’d known what I know now. I would have been a healthier me most of my life … You’re fighting yourself really, because you’re constantly in this yo-yo state.”

She sounds as if she is holding back tears when she says: “I don’t want to go back to what I was.” She is “90% secure” that she won’t. “My body is where it wants to be.”

Of course, there is an emotional adjustment to fitting a new physical form. She is more outgoing, happier, less angry. “I still have to come to terms with myself, that I have changed that much. People say: ‘Gotta learn to love yourself!’ When you first start out, you go: ‘What’s there to love?’ That’s how I felt. But I can look in the mirror now and not turn my head.”

Does Macdonald feel compassion for her old self? “Some,” she says. “But it was unnecessary to go this whole route before I finally coined on to ‘you can change’.”

I can’t help thinking that her desire for a transformation started long before she was 71. “In my teens, in high school, that’s when I started to struggle with weight. I thought I was really dumpy and fat, and yet I knew I wore clothes smaller than some of my friends who put me down,” she says.

Why did decades pass before she found what she needed to take her younger self in hand? “It took that many years to wake up,” she says. “Fifty-five years!” But maybe there is equilibrium in the way she came to these changes late.

As a girl, she says, she had an uneasy body image “because my development was really early. I resented losing my childhood. At 10 years old, you don’t want to not be a child any more.”

As a mature woman, she is powering her own rejuvenation. “People need to know, especially women, that you are not finished at 40, definitely not finished at 50 or 60 or 70. You can go on and on until the day there is no more,” she says. “And you should be able to do it with pizzazz.”

• Tell us: has your life taken a new direction after the age of 60?