It occurred to me, as I stared grimly at the clock on the treadmill in front of me, that this might be the only place where I was happy to see time pass. As quickly as possible. Especially when our endlessly energetic gym instructor urged us to "kick it up to a 16 per cent incline and run at your all-out pace for one minute in three, two, one!"

I focused all my attention on the digital display, which also served to distract me from the stupendous efforts being made on the treadmills to the right and left of me, and power-walked my way uphill.

Everyone has to start somewhere, my husband told me reassuringly, when I mentioned that I would be the weak link voted out if gym class were a reality television show. "Pack up your wet wipes and go."

Honestly, though, I wasn't fishing for comfort, having already reconciled myself with looking like the class underachiever at the gym I had joined in November - in a moment of weakness and wild panic when I realised my body was somewhat ahead of my mind in terms of its planned obsolescence.

Deny it as I might, it wasn't the weighing scale that was malfunctioning when it indicated the kilos were piling on without any effort on my part. Nor was it a quirk of the bathroom lighting that gravity seemed to be pulling the bags under my eyes towards land.

I had read that old age becomes most apparent when wrinkles appear on the neck and, sure enough, one morning my eyes settled, startled, on the loose stretch of skin below my chin that looked like creased paper. Gingerly, I pinched it. It crumpled and did not uncrumple when I removed my fingers.

Age is just a number, they say, so I tried it on for size. "I am 50," I declared. No, said my brain. No way. I know you can't turn back the clock, but you don't have to be happy about it. You can be dragged kicking and screaming into the next half century.

You can, indeed. As my mother- in-law is fond of saying, growing old is not for wimps.

Over the last few years, I have seen people I remembered as strong and in their prime really not that long ago become elderly and needing help. One's twilight years should be filled with the satisfaction of retirement and the blessing of grandchildren, but too often it is hollowed out by loneliness, financial stress and ill health instead.

Still, it was not for those reasons that I dreaded the passage of my 40s, but something much more mundane: vanity.

Here I was, nearly at my golden jubilee and still fighting that 25-year-old me who was worried about being overweight and ugly.

Worse, in fact. Now, I had to settle for what I could get: passing off as younger than I actually was. Oh, you look amazing (for your age).

Whether it's because of evolutionary biology, messages in the media or the relentless exposure of social platforms such as Facebook , it's hard to kick the habit of feeling good only because you're looking good.

Just like the svelte, 70something woman my friend, Andrea, overheard at her gym telling another woman recently: "I just want to get rid of this," pinching the little roll of belly fat just below her navel.

You'd think there comes a time in life when one might want to have different goals. Such as it's not the muffin top that needs to go so much as any mindset regarding what perfection means that sends us into our middle age and beyond under the impossible burden of looking as we did in the bloom of youth.

Blow us, as a certain actress playing General Leia Organa of the Rebel Alliance might say.

I finally turned 50 a week ago and regretted I had ever spent a moment worrying about it.

That can happen when you spend your birthday weekend with women friends who are accomplished, compassionate and wise, with whom you can take solace and joy in the freedom that the middle years can bring.

Really, is there someone you need to impress so much that you have to lose yourself? What I knew, instead, was that the world would be a poorer place without these beautiful, perfectly imperfect friends of mine.

Perhaps you really can learn to love the sorrows of your changing face, as Yeats once said.

Would I trade that peace for the heady bloom of youth? Not today. Ask me again next week.

I am not sure what I expected when I signed up at the gym with the impulsive thought of whipping my body into shape. I guess I thought I might gain a slimmer, fitter, new me.

But you know what? I am feeling so good with the simple fact of rigorous exercise a few times a week that I don't care all that much if I'm slimmer or newer or not.

How do my fellow gymgoers see me? As I see them, I guess - just another middle-aged woman working through her own private challenges - if they see me at all. A few years ago, I would have chafed at being invisible. Now, though, I'm okay with it.