It’s going to be a crowded, hectic day by the pool. The staff at reception look perturbed at the prospect but I arrive early enough to avoid a queue. Even at this hour the lanes are bustling and towels are being laid out on the decking for the sunbathers. Over the wall a mighty red-and-white tent has sprung up, as we are in the midst of the Brockwell Live series of festivals which have divided us all into YIMBY’s and NIMBY’s and caused no end of simmering fury in the street WhatsApp.

But there is no politics when your head is in the water, the light is shimmering around you, and there is only the metronomic beat of length, turn, kick, length, turn, kick. And when you climb out there is the unheated shower which at this time of year acquires some radiant heat from who knows where and becomes the perfect temperature for what will be a hot, sticky day.

It’s Mighty Hoopla at Brockwell Live today. The young folk are already gathering in glitter and make-up. May they dance themselves dizzy.

Today is that day. The lanes are very crowded indeed, because of people but also because there’s only one lifeguard on duty so half the pool is shut. There are only three lanes: a wide slow one for those seeking a gentle paddle and, more often than not, a good chat; a fast one for the crawlers; and the one in the middle, for those who refuse to be pigeonholed and/or have no one to talk to.

This middle lane, then, is a liminal space, with people swimming at very different speeds. It requires careful awareness and a willingness to overtake and be overtaken. It can also be alarming, as those in the fast crawler lane hurtle towards you in the opposite direction, hard up against the rope which divides the lanes, seemingly intent on running you down.

But we’re amenable folk, here in the middle lane. We let each other pass at either end. We overtake carefully and allow those overtaking us to pass without demur. We even, as happens this morning, make way for a child. We are, in swimming matters, determinedly and literally centrist.

When the rest of the pool is open, this non-aligned space will become less important. But this morning it’s the lane that holds the pool together. And so for that, my fellow mid-laners, I salute you.

And yet I persist in trying to fix things. One method is to book early (or earlyish) swims a week in advance, as if in seven days from now I will in fact be a different person, only to cancel them the day before when reality looms. The Lido rightly fines you for missing a booking, so I’m scrupulous about cancelling in time. And so the booking app turns into a permanent proxy war with myself.

But this morning I kept my appointment. There was a low mist over London, such that the Shard and Canary Wharf resembled the cover of a 70s space opera paperback. There was hardly anyone about. The Lido was bustling but not busting. And I had the best swim of the year so far.

I’ll now be booking early swims for the next week. And cancelling them every night. It’s too late to learn now.

On my seventh lap I find myself swimming up to a duck. It doesn’t seem to mind. In fact it barely moves out of the way at all, glancing at me with quite the glare of disdain. I can swim right around it, watching the sun gleam off its blue and green sides and seeing its feet flip diffidently beneath the surface.

It stays where it is for the remainder of my swim, but as I get out it pops over into the faster lane, a final act of derision for the slowsters. A crawler ploughing up and down the faster lane doesn’t even see it and the last image I have of the duck is it paddling furiously to keep ahead of the crawler, as if it too has decided to do more cardio.

By the time I walk home the bins have all been emptied. It’s nice to think that even now, after everything, someone is still emptying the bins.

Today’s swim has been something of a riposte to my previous post, which waxed blissful over the Goldilocks zone that is April. But April also has teeth. And though the sun is out and it is, by any measurement, a lovely morning, shoving oneself for up to 20 minutes in 14 degree water does leave one vulnerable.

Lesson learned. Or rather, relearned.

The water is clear, shining, exceptional. A duck clambers in ahead of me on my first length, the perfect distraction from the initial-immersion cold. Because it is so early the sun is low in the sky, so swimming from the deep end shallowwards is periodically uncomfortable – the sun straight in your eyes – and delightful – your eyes underwater seeing shimmering loveliness.

I climb out after 10 lengths, halfway to my target. I am tired, properly exercise-tired, for the first time in a while. I take a picture of the low sun and walk back home. There are more dogs now, and they look perkier.

Today is the apotheosis of the Goldilocks period. A handful of cheerful acolytes in each lane. Ribbons of light on the bottom and sides of the pool. Water so clear you can see your thoughts in it. Six weeks from now it’ll be clouded by sunscreen.

Working my way up to my customary kilometre after the winter break, I manage eight lengths. I could do more. But Goldilocks has a sting in her tail at this time of year. As my Fenland wife is fond of reminding me: ‘Ne’er cast a clout till May be out.’

Blue, blue, electric blue

On top of that, a pair of little saunas have appeared at the side of the pool. A queue of young folk wait patiently outside for their turn, though locking oneself in an overheated box with half-naked strangers has mysterious appeal.

Instead I immerse myself in cold water with half-naked strangers. Five days ago I was swimming in the Caribbean, sharing the water with locals taking a sea bath, jet skiers and, on one memorable morning, a (small, unthreatening, yet still primordially terrifying) shark.

So it is, by comparison, cold. It’s also crystal clear and astonishingly beautiful, the light rippling on the walls and floor of the pool in a way that I am starting to think might be unique to the Lido.

I am, I begin to feel, overdressed in winter swimming cap (about nine inches thick and made from a combination of car tyre and anorak), gloves and socks – and trunks, obviously, it’s not that kind of pool. On my penultimate lap I shed the gloves which is initially painful, like leaving my hands inside a freezer for half a minute, but soon the hands warm up too.

I exit and find my sliver of decking among the young sunbathers. Like every fifty-something person I possess the power of invisibility to anyone under 35, which makes changing easy. But as I am leaving I step over a reclining woman and my metal water bottle slides out of my rucksack and hits the deck with a shattering crash. I suddenly lose my invisibility and dozens of heads turn towards me. ‘What the hell is that?’ they seem to say. I give no reply and in a few seconds I am invisible once again. I leave them to their sunbathing, their phones, and their passionate debate about last night’s sushi.

The Lido is packed but the pool is virtually empty. Most people seem content to lie on the decking and catch the sun, and who can blame them? It’s been a hard winter, in all sorts of ways.

It is the time of year for screamers – young folk who step gingerly into the water, their arms folded pointlessly against the drop in temperature, and then yelp in distress as they realise quite what they’ve let themselves in for. We Old Ones smile indulgently and leave them to it.

It’s easier than yesterday, the swimming. The cramp in the shins eases off quickly, the water is clear and beautiful, and I manage to get my face in on the third lap. I finish at the end of the fourth, and take myself off to the decking to dry off and listen to the young folk making plans for Sunday coffee and pizza. I exit through the turnstile as another scream lashes out from the shallow end steps.

So now I must do it again – go in cold, literally. The day is glorious after a week of sun and increasing warmth. The water temperature has crept into double figures. This should be easy right?

Oh optimistic fool. No it is not easy. It’s never easy, the first time. As the Japanese don’t say about trees – the best time to start swimming again is at the end of the summer. The second best time is now.

So in I go. For some reason my legs always feel it first, around the calves and shins. It’s a kind of cramp. Then the chest begins to tighten. Then the breath starts to run away with itself. Then you’re at the end of the first length and giving active consideration to abandoning.

Then you turn for a second lap. For some reason, you start thinking about heart attacks. The sun is in your face now but it’s late morning so it doesn’t dazzle you, just inspires. And then you’re at the end of the second lap, giving active thought to a third before reminding yourself: it’s been 10 weeks. Take it easy.

I climb out. I’m alive – so alive (as cold water swimmers are contractually obliged to state). I sit in the arm sun writing this. I take a lovely picture. And I book tomorrow’s swim.