Serpentine Road, Hyde Park, W2; 020 7706 8114. Open daily, 8am to 9pm

In recent weeks, I have fretfully observed my increasingly middle-aged tendencies. The flat shoes, the creeping interest in ornithology, the strictness on matters of manners with other people’s kids, the switching between Radios 4 and 2. The feverish desire, not for cocaine, ecstasy or buckets of wine, but for a good gin and tonic with, urgh, nibbles. It’s a sort of madness, and it scares me.

This middle-aged awareness was brought into fierce relief by my joining the Serpentine Swimming Club. Twice a week, my recently redundant friend (the recession is all good for me — so many more people to kick around with by day) and I have been taking the duck-poo-flavoured waters of London’s loveliest lake. While there are athletic young members, largely this is a hearty and grown-up thing to do. For a start, you have to be in and out before 9.30am. We have yet to strip down to the ancient Speedos, worn gossamer-thin and baggy by the decades, and a rubbery floral swimming hat, which is what the hardy old-timers wear. We still prefer the young wimp’s kit of wetsuits.

Honestly, swimming in the Serpentine feels like heaven. This morning, as we walked to breakfast at the new Serpentine Bar and Kitchen, after our 4x400-metre laps, we marvelled, as a Country Life sort might say, at how many breeds of duck and goose you can swim up close and personal to. (It’s only a matter of time before we turn up with a worn Waitrose carrier full of stale bread to feed them with, like proper mad old bags.)

Did you see the heron? Isn’t the water clear? Same time tomorrow? The slide into middle age feels less Carol Ann Duffy, more John Betjeman. I’ve been having to self-medicate with large quantities of techno by people called Adam Freeland, Butane and Futureshock, a silly new car and some questionable fashion choices.

Actually, it’s pretty cool being middle-aged. Listening to Fleetwood Mac all the time isn’t such a crime, and swimming in the Serpentine is the second-best thing you can do in London with your clothes off, and definitely the best thing you can do while wearing rubber. My swim buddy and I felt real tea-shop joy when the Serpentine Bar and Kitchen opened. It’s in the old Dell restaurant, at the eastern end of the lake, in front of the rose garden, which is about to come into fragrant bloom, and is, in a sweet urban-romantic way, eternally popular with lonely chaps looking for “someone to hold” after dark.

The restaurant was designed by the Welsh architect Patrick Gwynne and built in 1965. Its roof is like a bird in flight, and its west-facing aspect is entirely glass, so even if you sit in, you feel like you’re out. It is unmistakably Modern, with a capital M; Prince Charles Probably Wouldn’t Approve, but thankfully, while the Windsors may own the royal parks, he can’t be knocking all the interesting bits down and replacing it with dull retro follies.

The view is achingly lovely, over the lake to the Serpentine bridge. We both glowed with love for London’s parks. My friend liked what they’d done inside: gone is the airport-lounge vastness of the old cafeteria and, in its place, wood panelling, shades of aubergine and muddy lime, and the tatty-chic muddle of country-kitchen type furniture, which, frankly, I’m a bit sick of after two years reviewing gastropubs for a London paper. I thought the inside was a bit discordant with the out; she, I admit grudgingly, is a sleeker sort than me, so I bow to her wisdom. The floral bunting really was a bit much, though, and I felt as if I was sitting, grumpily askew, in a Boden catalogue. Brutal modernism can be chair-scraping noisy, and the new management, Benugo, has softened the joint and made it welcoming. Cornflowers, rosemary and wild strawberries in pots, alongside trays of proper moutarde, English and dijon, HP, ketchup, Tabasco and Lea & Perrins, on each table are a nice touch. Everything’s served on Burleigh crockery, which is just the right side of twee.

So we ordered good leaf tea and above-average, but not brilliant, coffee, and set about the important business of breakfast. The menu by day and night is largely British and seasonal, with Italian touches: fishcakes, pea and asparagus risotto, cold leg of lamb with haricot bean purée, rhubarb and sherry trifle. I love the fact that they have a daily roast meat sandwich (today’s was pork belly). But breakfast is my favourite meal, especially when it’s in the park after swimming.

We shared a bircher muesli, a very middle-aged delight, I suppose, which is oats, fruits and nuts soaked overnight, and a drastic improvement on porridge. It was good, full of goodies and the milkiness that comes not from dairy, but from the oats’ natural creaminess. My friend, with her Teutonic roots, said it was technically perfect, if a little sweet. I’d ask for it without honey next time, although they’re strict on not allowing princessy, off-menu ordering during busy times. Not that the staff were rude. They were charming, all booted in uniform Dunlop Green Flash — you’ve got to love that touch. There are other great details: Scrabble, a well-stocked bar with a considered selection of international beers and British ales, and only the poshest spirits, including Patron tequila, Krupnik honey vodka and proper Plymouth gin. There’s a baby grand in a far corner, and anyone’s allowed a pop as long as they can actually play.

I will definitely be back for a gin and tonic on the terrace; it has to be the perfect spot for a summer birthday champagne breakfast. The wine list was good value, but a bit narrow and dull. I showed it to the more expert Tom Harrow at Wine Chap, and he agreed. “Beige and unexotic,” he called it.

We started off happy with the food, but then it, too, fell a bit flat. The scrambled egg in swim-bud’s Serpentine breakfast “had the perfect runniness, creamy and yummy”, the grilled tomato was orange and watery, the black pudding was a bit on the claggy, bready side, the good smoked back bacon felt as if it had been sitting around drying out, as did the baked beans; a proper herby, meaty sausage rescued it a bit. I had a variation on eggs florentine with purple sprouting broccoli, which runs a close second, after asparagus, as the pride of Britain’s soils right now. The barely seasoned dish didn’t fall together around the stalky veg; it was not an improvement on spinach, and the hollandaise sauce lacked an essential richness. Still, at £7, I ain’t complaining. We had a super-happy time for £20. Which is, coincidentally, all it costs to swim between 6am and 9.30am every day of the year as a member of the Serpentine Swimming Club.

We bunked off the bunking-off of freelancing for the morning. “I’m embracing middle age,” my swim buddy said as we walked the two miles home in our sensible shoes, talking about how much we loved ducks and Botox.

AA Gill is away

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