It’s 5am in Docklands in London and as the City slumbers there are few signs of life. Even the glistening towers of Canary Wharf have yet to wake up while the surrounding roads are deserted. But tucked incongruously at the heart of this sleeping cityscape one place is very much awake. Billingsgate is the UK’s busiest fish market with 25,000 tonnes of fresh produce passing through its doors each year. Entering the cavernous market hall we are thrust into bright lights, chattering traders and fish as far as the eye can see, all laid out across acres of ice.

I’m staggered by the size of the place, but my companion for the morning, the Michelin-starred chef Tom Aikens, is appraising things more coolly. It is his territory after all. “This way,” he says after surveying the scene like a general about to go into battle. And we’re off, dodging porters’ trolleys and the streams of traders carrying crates through the gangways between the stalls. As we move, I learn how to spot the good from the bad. “A lot of the fish here is imported,” Aikens says. “Some is a week old and not all of it has travelled well. If the eyes are sunken and the flesh is dull, it’s already past its best.”

He carries on, rattling off the names of fish I wouldn’t recognise if they were labelled in an aquarium, pausing occasionally to question stallholders. Finally, he settles on some sea bass, plump, shiny and beautifully fresh. Stuffing this into a rucksack, we head for the exit and start running again, because this is a shopping trip with a difference — having already run the nine miles here from Aikens’ eponymous restaurant in Chelsea, we’re running a marathon-sized lap of London’s produce markets to collect the ingredients for the ideal post-marathon meal that Aikens will cook on our return.

Waiting for Aikens at his restaurant a couple of hours earlier, though, I had wondered if he’d show up at all because he returned home only two days ago after completing the Marathon des Sables, a 150-mile-long, six-day race across the blazing heat of the Sahara Desert. “I push myself at work, and release that pressure by pushing myself in sport,” he says as we run that first leg to Billingsgate. “My life is always full-on — I’ve never been a person who’s happy sitting back watching soaps”.

Full-on is a good description. Now 39, Aikens made a meteoric rise through the ranks via tough stints under some of the best chefs in the world to become the youngest British chef to win two Michelin stars before opening his own restaurant, called Tom Aikens, in 2003. The awards and rave reviews poured in, making the restaurant, and its chef one of the country’s hottest culinary properties.

But in 2008 the credit crunch bit into his business model and Aikens attracted headlines for all the wrong reasons as suppliers left badly out of pocket complained very publicly. An equally public repentance was followed by Aikens getting back to what he knows best: hard graft and superb cooking. The plaudits are coming back while the stars never went away — the restaurant has held its one-star status since 2004.

Back on our marathon trail, the traffic’s picking up and the route into town from Billingsgate is nose-to-tail with vehicles. While most drivers are opting for the Limehouse Link tunnel, it looks like a fume-filled hell for runners, so we take the open air of the A13, which funnels us into the City. We’re trading on the markets too this morning, just not the financial ones and as the City fades behind us we approach Smithfield meat market, but first we cave-in to the tempting wafting coffee aromas of the area’s cafés, all preparing for the breakfast trade. Caffeine fix out of the way, we slice through Smithfield in full swing because to make sure this feast is perfect for marathon finishers, the sports nutritionist Lyn Clay has advised Aikens on the nutritional qualities the meal needs, and recommends avoiding meat.

“You need 20g-30g of protein per person after a marathon,” she says. “Beans or meat can provide this but aren’t ideal on your stomach. The best thing is fish.”

Hence the sea bass already bought: now we’re going to Borough Market for fruit and veg. “I used to come here years ago when I first came to London and it was very different,” Aikens says. “It was just a fruit-and-veg market for the trade, but now there’s a bit of everything and it’s more of a tourist attraction. It hasn’t lost any of its charm though.”

Before shopping at Borough, however, our tastebuds are distracted again. This time the culprit is toast and jam — with 15 miles run, carb supplies are running low. Simple, sweet and hot, the toast hits the spot perfectly and, suitably refuelled, Aikens prowls the market to search out the rest of his ingredients.

Purple sprouting broccoli goes into the bag first because it’s “bang in season at this time of year and more tender than regular broccoli”. Kale follows — “greenery, good for us, plenty of iron and in season too” — as does spinach, garlic, cherry tomatoes, potatoes and banana shallots, which Aikens favours as “they’re bigger, so they’re easier to chop”. This lot won’t only taste good but will also fulfil Clay’s criteria for post-marathon iron, vitamins and antioxidants to boost the immune system, which can be weakened after running. Finally, blood oranges, redcurrants and mint are picked up for an extra antioxidant and vitamin-boosting dessert. Then we’re running again, back to Chelsea. I feel out of place wandering into one of London’s smartest restaurants in sweaty running gear but as Aikens has no qualms I follow his lead and dive through the plush richly carpeted silence of the empty dining room — lunch is still several hours away — and descend to the kitchen below. Here opulence is replaced by an almost military austerity. Aikens is in his element and as he dices, slices, chops, simmers and grills, instructing staff, tasting sauces, soups and dishes, and either nodding in approval or advising on improvements.

Half an hour later, two plates of post-marathon perfection are ready with dessert and, now starving, we retire to the private dining room. Passing the restaurant’s vast and deeply-stacked wine cabinets on the way, a glass of something crisp and white is tempting but is sidestepped in favour of water as dehydration is still high after our run.

The main dish is simple, yet sublime. The soft-grilled sea bass blending with the greens and mash while the tomato, garlic and shallot sauce adds a subtle dash of vigour. As my hunger subsides, I can feel my body gratefully soaking up the nutrients. The zinging fresh blood oranges, mint and redcurrants with a dollop of crème fraîche are also delicious.

Clay says: “The worst thing you can do after a marathon is stuff down a pizza, a takeaway or a ready meal. The best strategy is a recovery drink at the finish then feeding yourself properly as soon as possible. Your body will thank you for it.”