When Celia Powis hit 70, she was accused of being too old to teach her fitness class. But she has proved she can still give the youngsters a run for their money. Sussex's most spritely pensioner put our girl, Jessica Bauldry, 28, through her paces.

Age has never been an issue for Celia Powis.

So when the 70-year-old grandmother was sacked as a fitness instructor because she was too old, she stood firm.

The mother-of-three won her fight with Worthing Leisure Centre and returned to work hours afterwards.

A week later I was persuaded to try one of Celia's celebrated workouts.

I was confident I had the advantage of age and had completed the Sussex Beacon Half Marathon last month.

But I felt knackered after a long phone interview with the energetic instructor after she revealed she had been teaching aerobics for 32 years. At the peak of her career, she was taking 15 classes a week.

She told me: "I fell into it, literally.

I went to movement by music at Worthing Leisure Centre and one night the instructor was ill and asked me to take a session.

Then I took on the class permanently and made the sessions up.

A year later I found popmobility, which was one of earliest forms of aerobics. I introduced it to my class and it just escalated from there."

As a result of her fitness regime, Celia is rarely ill and, unlike a lot of women her age, has healthy bones and joints.

Celia's mind is just as trim as her body. She knows more than 600 fitness routines devised by the popmobility team in London.

Celia told me: "I'm just lucky I have that sort of brain."

Meeting Celia in the flesh had me quaking in my leg warmers.

She now teaches only seven classes a week but the toned and tanned grandmother is a picture of youth.

As I looked down at my muffin top, I found myself envious of the physique of someone more than twice my age.

Celia is a walking advert for her fitness programme.

But you do not look that good without a bit of pain as I soon learnt at her class at St Richard's Church, in Worthing.

Popmobility is designed to work every part of your body in imaginative, dance-like movements.

It is meant to be fun but involves a lot of concentration and co-ordination.

Celia, who attracted 400 people to her classes at her peak, began teaching as a young mother. A lot of her customers are mothers.

"We all realised when we had children that you have to have time on your own," she said. "Popmo is perfect for that.

"It's a marvellous way to get completely out of yourself. You can't think about what you're going to cook for dinner that night because you have to follow the movements so closely."

At the same time, I think a large part of the appeal is Celia's personality.

I struggled through the warm-ups, but she is not one of those frighteningly ferocious instructors who tells you to "enjoy the burn". She was enthusiastic, gently encouraging and sang along unselfconsciously to the music.

Even as the pace increased, she kept it up. I was determined not to be beaten and threw myself into the routines.

If Celia could touch her toes and do dozens of press ups, so could I.

The first time I looked at the clock, I realised I was only 15 minutes into the one-hour session.

As sweat dripped over my 1980s style visor, I pushed myself harder. Celia was right about the complex movements taking you out of yourself. I had no time to think about work.

I tried not to look at the clock but as my flabby limbs flailed, I wanted the class to be over.

Celia was barely breaking sweat. I looked up from my pained position and she was flexing her muscles victoriously. I wondered if I had found the inspiration behind the Duracell Bunny.

To make things worse, I caught sight of a wild-haired, red-faced woman reflected in the window and realised it was me.

Too much fast-food and sitting at a desk all day was poor preparation for a workout with a legend like Celia. I bumbled pathetically through the last routines before giving up with cramp in my foot five minutes before the end.

I was impressed by Celia's steely stamina but what surprised me most was that the rest of the class was barely out of breath.

Shirley Watson, 54, who has been coming to the same class for 30 years, said: "It's a hard workout. A lot of people think it looks easy but it isn't. The more you come, the more you get used to it. Celia's also such a unique person. I've made so many friends here it makes it really enjoyable.

Lisa Wright, 55, who has been a customer for 30 years, added: "I've tried lots of other aerobics but the key to Celia's class is the variety. It's not repetitive or boring. There are so many routines."

To watch Jess being put through her paces by Celia, click here