I have a long and cowardly relationship with swimming. Not for me any deep-sea heroics with sharks off the shores of Malaysia, or snorkelling around shipwrecks. To my shame, I confess I have never swum in the sea; I have never dived. I have always waited for at least an hour - more like two - before taking to the water after a meal, and that water has always been the artificially turquoise blue, fragrant with chlorine, of the safely enclosed indoor swimming pool.

Yet I really do love swimming: swimming is the essence of freedom - just so long as I'm not out of my depth, and it's a quiet Sunday lunchtime, and all the squawking children and burly dads have hauled themselves out of the water for a clubroom lunch in front of the rugby. Then, with no one cluttering the lanes, I power happily up and down, back and forth, 70 times. It has to be 70, even though 64 lengths count as a mile. "Going beyond" is one of our university's mission slogans, so that's what I do. I go the extra (bit of a) mile. Why stop at 64 if you have it in you to do 70? But equally, why commit to 72 if you might have to do that every week from then on?

Swimming brings out the worst in me, and not only these habits of obsessive-compulsive behaviour. Over the years it has made me tell lies, deceive teachers and push past the dreamy, drifting mums with one eye on the soup of children bubbling beyond the "slow" lane; I who have myself been repeatedly overtaken on the drive to the pool by people impatient with my self-righteous adherence to the speed limit. Driving and swimming are the yin and yang of my weekend life, sharing a common history of early, long-drawn-out avoidance. I learned to do both and then pretended I couldn't do either; then, when I was ready at last, it had to be on my terms - no diving in pools, no driving on motorways. It was of a piece with not walking until I was two (I had seen my younger brother do it, and knew the risks). Now I love it - walking, that is, and running, and Pilates, but not driving on motorways. I love all exercise, but have never learned how to do any of it properly. I have tight hamstrings, I don't like putting my head under water, I have never mastered the crawl or the butterfly, and I swim with my head up, like a moorhen, as people of my age do, because that was how we were taught - but at least I don't walk in the water, which seems to be the latest variant on not swimming when swimming is what you're purporting to do.

Since when did walking become the new swimming? I long to stop and ask them, those pairs of fat friends who amble up and down the lanes, as out of place as any leisurely couple taking a stroll up the central reservation of the M25. Sometimes they vary it and walk backwards to up the challenge, or else they sneak in a normal length of breaststroke before scuttling past through a backwash, elbows jutting outwards, in an aqua power-walk of unexpected velocity. Perhaps, by keeping their feet on the pool floor, their eyes on the distant horizon, they too are avoiding the horrifying realities of actual swimming.

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Like most people, I began swimming as a child. Both my parents thought it was a good idea, although my mother hated it herself and never learned to swim properly. The smell of slippery duckboards mixed with chlorine gas of trench-warfare intensity was enough to put her off, combined with the usual swimming-bath experience of muffled shouting heard through waterlogged ears. The last straw, she tells me, was returning to her changing cubicle to find a wet footprint on her liberty bodice, thrown carelessly on to the floor. Things were only slightly more advanced in my own childhood, my school hiring the local orphanage (of all places) for our weekly shivering attempt at the basic strokes. Knowing that the orphanage was for the children of drowned trawlermen did little for my confidence, and when the caretaker forgot to turn on the water heater (which happened fairly often), it was all too much like the Barents Sea to tempt me into any displays of aqua-bravery.

My father, who never showed off and was what I would call a "rational swimmer", sensibly tried less threatening options until I finally let go of the ropes and paddled away in a sunny hotel swimming pool in Bournemouth. What made all the difference was the sweep of broad, shallow steps into the pool. You could inch your way down at your own pace, and even sit on the steps for a while without being pushed. You could wait until the noisier, splashier swimmers got out and went to the hotel lounge for their tea and chocolate eclairs. Yet even here, chaperoned by sunbathers, there was no absolute guarantee of personal safety. I looked up once to find my father (a medical practitioner, never entirely off duty) trying to resuscitate a middle-aged man to whom something dreadful had happened in the pool. Perhaps he had gone in without waiting the full hour since eating, on which my father always insisted. Nevertheless there was our Dad, doing his best to save the man before the arrival of screaming ambulance, paramedics, stretchers and high drama.

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"You do realise that man died," my brother told me decades later, when I was preparing a speech to toast my father's 80th birthday.

"No, he didn't," I insisted, "Dad saved him."

"I think you'll find he didn't," my brother insisted back. It was a good job I never knew this at the time: it would have set me back another 10 years. Even in a state of trusting faith, it was one more haunting shadow cast over the lovely flickering turquoise water of the summer holiday swimming pool.

Fathers nowadays are much more gung-ho around the water, flinging their children about like wet cloths or dropping them from a height while the pool attendants watch, yawning. Miss Trunchbull, in Roald Dahl's Matilda (1988), hurling children by the pigtails, has nothing on these macho tattooed dads determined to toughen up their kids and make them confident in the water. The strange thing is that it seems to work. For every whimpering child still clinging to the edge, there seem to be 10 howling for another round of tossing and dunking.

I wait patiently until 12.30pm, the "no unattended children" hour, when I can cruise peacefully up and down, planning my week or reflecting on the one just gone. If the long, slow, hot bath is a boon to academics wanting space and time to mull over the next book or meeting, how much more wonderful is the whole spacious vastness of the pool on a Sunday lunchtime? There is nothing more disturbing than the lapping of indoor waves, or the gentle splash of other passing philosophers as we oar up and down, backwards and forwards, thinking of books and relationships, shopping lists and the night before. If the pool is the bath writ large, the thinking should be phenomenal, even if it amounts to nothing greater than the planning of a party, or the last elusive sentence of an article. If the price often paid is sudden, frightening amnesia as to the number of lengths completed (was it 58 or 60?), it at least gives me the opportunity to forgive and forget. (I'm sure it was 60: and anyway, who's counting?)

Having decided some years ago to abandon the horrors of the municipal pool with its dive-bombing flabby 14-year-old boys, I joined a private health and fitness club, where you can relax in the Jacuzzi, the sauna or steam room, and drink skinny cappuccinos in the clubroom afterwards. While this puts a whole new gloss on swimming (no more greasy duckboards and rubbery mats, no more shivering in smelly tiled corridors), it introduces new social challenges that never existed in more primitive times: chiefly the etiquette of the shared space, the enforced intimacy of the circular Jacuzzi pool. I would rather wait for weeks to have the Jacuzzi to myself than break, uninvited, into the party.

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In any case, how can people sit there, with the bubbles fizzing around them, in a seminar-like formation, without feeling they ought to be in a consortium, or at the very least a collaborative networking bid? What a great idea for an awayday, although for this and other reasons I remain thankful that my university has so far declined to build its own swimming pool. Imagine finding a departmental meeting, almost quorate, collectively chalking up its 64 lengths at the end of a day's marking:

"Well, I'm sorry, but I thought 68. You can't give a first to someone who can't spell 'Foucault'."

"Sorry, my ears are bunged up. I can't hear you..."

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As it is, some of my faculty colleagues do occasionally get in the pool, towing their children about. We wave shyly or pretend not to recognise each other without glasses (or most of our clothes).

Swimming is not my only sport. When the English department fielded a running team for the annual cancer charity Race for Life, I began by sponsoring and then joined in. To my surprise, I found I had the stamina to run the 5km circuit round the city centre, happily window-shopping as I went. When it moved to a real running track, with nothing to look at except more track and rows of Portaloos, it felt rather less cheery. Next came Pilates, hampered by the hamstrings and by teaching commitments immediately before class. You need a mat for Pilates, which I now smuggle into my room on Wednesdays, sneaking in the back way and flinching from potentially embarrassing questions about afternoon naps or urgent camping holidays. With scarcely time to change, the trick is to go to work in almost the right kit for "ab curls" and deep breathing; I then scramble into leggings and bolt down the road, mat banging against my back, to burst breathless into the church hall where my classmates are already flexing their feet at the ceiling, or - as sometimes happens - doing "dry swimming", lying flat on the floor and feigning the right movements without the advantage of any actual water.

Real swimming is easier, even if it has taught me more about shiftiness and evasion than manning up; but having devoted most of this essay to pool-related neuroses, I realise I now have only a paragraph to say something positive. Well, the joy of swimming can be summed up in the difference between Sunday morning and Sunday afternoon. If Sunday morning is often spent in lacklustre contemplation of the computer screen, tired and headachy, Sunday afternoon, post-swimming, is full of endorphin-fuelled joy and vitality. However unenthusiastic I feel about that moment of lowering myself into the never-quite-warm-enough water, two lengths later and it all comes right. There is nothing so energising and relaxing that also makes you feel so virtuous and hungry. You can eat anything afterwards, anything at all, and after one last burst of work you can lounge on the sofa for the rest of the day, knowing that whatever happens in the week to come you have done what you can for mind and body.

If walking is the new swimming, swimming is the new thinking, and if they would only play some Mozart over the public address system, there would be nothing further to wish for.

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