It's one of the greatest walking days in Scotland, on hills unmatched for scale and arctic grandeur; but however familiar you might be with it, the round of the Cairngorm giants is no pushover, as Dan Bailey discovers.

With a thick cushion of daylight, and at least the theory of some half-decent weather to enjoy it in, mid summer is high season for a walking epic. Cramming a route that you might usually backpack over two stages into a very full day trip has the added advantage, at this time of year, of sparing you a high camp among the midges. And if you're going big, you can't do better than a round of the 4000-foot Cairngorm peaks.

Though founded on an Imperial height that no one under the age of about 60 would otherwise ever dream of using, the Munro bagging scene does at least have the logic of a nice neat 3000 figure. Lording it over the crowds of more lowly Munros, the 4000-foot summits are a rarefied subset, a club with only nine members.

Lochaber's four might be the more rugged group, and form the meat of another classic big traverse; but the five four thousanders grouped together in the northern Cairngorms beat them both for wildness and sheer mass. Comprising the second, third, fourth, fifth and sixth highest mountains in Scotland, these sprawling peaks account for the overwhelming bulk of the country's high ground, and a day spent up on the plateau among the snow patches and croaking ptarmigan takes you as close to the feel of the Arctic as you'll get this side of Scandinavia.

Divided into two massifs staring each other down across the deep trench of the Lairig Ghru, the Cairngorm biggies would typically be done over two separate walks - both pretty big days in themselves. But the scale of these hills really suits an ambitious approach, and the all-in traverse of the Cairngorm 4000ers is a high mountain journey of unique atmosphere and stature.

It's not my first time, but something this good bears repetition. I last did it clockwise, so to mix things up I'm trying the reverse today, setting off at a civilised 8:30am from the Sugarbowl car park and heading first in the direction of Braeriach. The idea is to get that bouldery up-and-over through the Chalamain Gap out of the way while the legs are still bouncy, and before the day's forecast sunshine gets too warm. But I should have set the alarm for sparrow's fart, and already I'm sweating. Living in the highlands, anything approaching high teens is classed as sweltery.

I pass a couple of mountain bikers crawling their bikes uphill on a decent path, and haven't the heart to warn them that things are about to get harder. Cut by glacial torrents, the now-dry Chalamain Gap is a quirky little gulch, choked with piles of granite boulders that make it a bit of an obstacle for the two-wheeled or heavily laden. Today I'm armed with only a light running pack, and though I've planned to maintain a steady stride at walking pace to eat the distance without knackering myself, I can't help breaking into a trot on the easy downhill from the western side of the gap into the base of the Lairig Ghru.

There's a moment of decision: bin the attempt, or press on and hope to regain enough mojo to finish

Glacier-carved into the heart of the range, this huge pass is Scotland's most dramatic through-route, flanked with the screes and rotting crags of the Cairngorm giants, and coming to a wild and windy high point that overtops most of Britain's mountain summits. A circular link-up of the Lairig Ghru and its neighbouring pass the Lairig an Laoigh makes a memorable challenge walk in its own right, and one we covered a few years ago in our Big Routes series:

But far from being a convenient line of weakness through the high ground, today the Ghru is more of a hurdle, and on my 4000ers round I'll have to descend into it and climb back out twice. I'm really feeling the heat on the steep trudge up Braeriach's long northern shoulder. Spending a recent day in bed with a non-covid cold probably isn't the best preparation for an endurance test; the energy levels seem ominously low and I'm suddenly having doubts about the whole enterprise. Stopping to have a word with myself, and a few jelly babies, seems to do some good, but the pace will have to slacken if I'm going to last the distance, let alone actually enjoy it.

Perched on the edge of the crags overlooking the scoop of An Garbh Choire, the monumental corrie complex shared between the western three 4000ers, the summit of Braeriach is as splendid as ever, and there's a bit of breeze now to help keep me enthused. It's easy progress for a while, across the gravel-strewn expanse of the Braeriach plateau, crossing the snow-fringed birth waters of the River Dee and following the corrie edge around clifftops where deep blobs of summer snow still fill the shady gullies. The largest area of properly high ground in the country, this is a serious place to be in grim weather or poor visibility; but with skylarks larking and views spanning half of Scotland it's a joy.

A nice bit of ridge takes shape on the down-and-up to Sgor an Lochan Uaine, and having passed a few folk in the intervening kilometres I've got the summit to myself. The other four peaks of the round are all well seen from here, but the dome of Cairngorm, my eventual destination, looks dauntingly hazy with distance.

Cairn Toul is all painstaking rock-tottering, and as I commence the long descent back into the Lairig Ghru the sky takes on a menacing darkness and scattered droplets fleck the air. If the rain comes down and the wind picks up it'll feel a long way home over the Macdui plateau, and my decision to save weight on waterproof trousers could yet be proved a rash economy. By the time you've reached Corrour Bothy in the base of the Ghru you've lost a dispiriting amount of height; and it's all to regain - plus a bit - between here and Macdui's summit, which looms tauntingly above.

Several walkers are heading south on the main track through the pass, aiming for an afternoon finish at the bothy. But on paper I'm basically only halfway through my day - and still not really firing on all cylinders. The moment of decision comes soon after; bin the round for a defeated trudge home along the Lairig Ghru, or press on with plan A and hope to regain enough mojo to see it through. Cairn Toul's top is sliding into grey fuzz, but the sky is brighter behind, and threatened rain has yet to materialise. The weather isn't giving me an excuse. Besides, here in the wild heart of the Cairngorms there are no easy options, and home is a long haul whichever way you get there. Night will be a summer dim, hours from now, and since it doesn't matter what time I finish or how slow I plod, I may as well go for it.

Fuelled on cereal bars and an old gel unearthed from the bottom of the pack, I get stuck into the long uphill grind. It's a bugger of an ascent on tired legs, at first on a decent path by the gurgling Allt Clach nan Taillear, then a stony spur that seems to have no end - until you suddenly find yourself up on the Macdui plateau. Gloved and hatted in a gusting wind that could have blown straight out of November, I've finally hit my stride, and I'm soon over Macdui's empty summit and trotting north across the rolling wastes in steely light.

An advantage of the anti-clockwise approach is a finish on comparatively easier ground, and though the summit of Cairngorm is still a fair bit more than a mere formality, the end is no longer in doubt. Ten hours walking; no records threatened, perhaps, but not too shabby in the circumstances. It's a route to savour, not to hurry, but I can't help wondering how much faster I might be next time.