A 2.4-mile (3.8km) open-water swim before 112-miles (180km) on the bike and a 26.2-mile (42.2km) marathon run. Or the equivalent of swimming from London Bridge to Big Ben in the Thames, cycling to Bristol and then running down to Glastonbury. In less than 17 hours. There’s a reason an Ironman triathlon is seen as the ultimate one-day endurance sport challenge.
And it was an ultimate Ironman challenge in Kona that former racing driver Billy Monger took on for Comic Relief 2025, completing the 2024 Ironman World Championship event in a record time for a double-leg amputee by crossing the hallowed Ali’i Drive finishing line in 14 hours.
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Ironman is the major organiser of these long-distance triathlons around the world (and holds the copyright to the name), but there are many other race organisers offering Iron-distance events, some that offer courses for beginners to Iron-distance racing (see the UK Ultimate Triathlon and Outlaw in the UK) and others at the extreme end of Iron racing.
There are also a wealth of shorter triathlons for athletes to enter, including sprint, Olympic, middle-distance/Ironman 70.3, as well as off-road triathlons, run/bike/run duathlons, swimrun, aquathlons, aquabikes and more.
When did modern-day triathlon?
Modern-day triathlon began in San Diego in 1974 after a debate between single-discipline athletes about whether swimming or running was the toughest sport. The first Ironman Hawaii was held in 1978 and it’s still the sport’s flagship race, the Big Island of Kona becoming the Wembley, Wimbledon and Lords of triathlon.
What are the triathlon distances?
These are the standardised distances in kilometres (the usual metric for listing race distances), but there can be differences at each event:
- Super-sprint-distance – 400m swim/10km bike/2.5km run
- Sprint-distance – 750m swim/20km bike/5km run
- Standard/Olympic-distance – 1.5km swim/40km bike/10km run
- Middle/Ironman 70.3 distance – 1.9km swim/90km bike/21.1km run
- Ironman – 3.8km swim/180km bike/42.2km run
The UK's toughest triathlons
Slateman Ultra, North Wales
Llanberis, Wales. 8 June 2025
2.4-mile swim/112-mile bike/31-mile run
Not content with 140.6 miles of pushing competitors’ minds and bodies to the limits, triathlon organisers are now offering ‘extreme’ Iron-distance challenges, of which the Slateman Ultra in Eryri (Snowdonia) is the UK’s newest and toughest.
The action begins with a 2.4-mile (3.8km) swim Llanberis’ chilly Llyn Padarn in the shadow of Yr Wyddfa (Snowdon) before the bike course takes on Pen-y-Pass not once but twice on its 112-mile (180km) journey with 2,679m of elevation gain. But it’s the 31-mile (49.9km) run course where completion dreams will be dashed or realised, taking on the zig zag climbs within the dramatic Dinorwic slate quarry UNESCO World Heritage Site before venturing up to the summit of Yr Wyddfa and its limb-shaking descent back to the Llanberis finish line.
There are also less hardcore Slateman events over the weekend, including sprint- and standard-distance triathlons and duathlon options.
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Celtman, Scottish Highlands
Wester Ross, Scotland. 14 June 2025
3.4km swim/200km bike/42km run
Also extreme is the Celtman, which takes on the Torridon Mountains in the Scottish Highlands over its gruelling 245km route.
If battling the jellyfish on the 3.4km (2.1-mile) swim and the windswept 200km (124-mile) bike leg weren’t enough, the Celtman then raises the ante with one of triathlon’s toughest 42km (26.2-mile) marathon run legs. The route takes in the rocky spine of the Ben Eighe munro (useful if you're into munro-bagging) before ascending to its summit, with a rock-filled scree chute to navigate on the way down.
Ironman Wales, West Wales
Tenby, Wales; 21 September 2025
2.4-mile swim/112-mile bike/26.2-mile run
Ironman Wales is a classic of the calendar, with a beautiful yet often choppy 2.4-mile (3.8km) sea swim in Tenby, Pembrokeshire, before a rolling 112-mile (180km) bike route in Pembrokeshire and a raucous seafront marathon run.
With over 2,000m of climbing on the bike course alone, it’s one of the toughest Ironmans in the world, with the spectator-packed climb at Heartbreak Hill entering Ironman folklore. What also makes this so memorable is how the Welsh town of Tenby truly welcomes the Ironman, producing a supportive spectacle from daybreak until the 17-hour cut-off.
The Restless, Argyll and Bute
Arrochar Alps, Scotland. 23 August 2025
1.8km swim/50km bike/25km run
From the same organisers as Celtman is The Restless, a new gravel cycling triathlon in Scotland’s Arrochar Alps, north-west of Glasgow, that concludes with an ascent of Ben Donich.
It might be midsummer, but the water temperature in the deep Loch Goil will still be chilly for the 1.8km swim. The 50km bike course is designed specifically for gravel bikes and boasts a mighty 2,000m of elevation gain over its route, including rocky and steep singletrack sections that will reduce many riders to pushing their bikes.
With 1,200m of ascent over its 25km route, the run leg goes relentlessly uphill to the peak of Ben Donich… and back down again on jagged mountain tracks. Prepare for jelly legs and the most refreshing can of finish line Irn-Bru in history.
Helvellyn Triathlon, Lake District
Glenridding, Lake District, Cumbria; 7 September 2025
1-mile swim/38-mile bike/9-mile run
Since its inception in 2004, the Helvellyn Triathlon has attracted the toughest of triathletes (including triathlon’s most famous stars, the Brownlee brothers) to Cumbria, armed with the mandatory compass, whistle, map and survival blanket.
The long-standing race tasks entrants with a mile (1.6km) swim in Ullswater, a 38-mile (61km) cycle up Kirkstone Pass and a nine-mile (14.5km) fell running route that goes within touching distance of the titular Helvellyn peak, before a toenail-troubling descent to the finish line.
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Norseman, Norway
Norway; 2 August 2025
2.4-mile swim/112-mile bike/26.2-mile run
Every August, Norway’s dramatic west fjordland region is the setting for the Norseman Iron-distance event that’s become one of the most infamous and influential in global tri since its launch. The epic grandaddy of extreme Iron-distance events commences with a leap from a ferry into a teeth-chattering Norwegian fjord for the 2.4-mile (3.8km) swim, before giant climbs and a constant battle with the elements are key features of the point-to-point 112-mile (180km) bike course. The peak of Gaustatoppen is the target of the marathon run – often in treacherous weather conditions – and is where the race’s iconic black t-shirt awaits all finishers. Almost as tough as the race itself is getting a starting spot: over 6,000 triathletes applied for the 250 entries in 2025.
Ironman World Championships, Hawaii
Kona, Hawaii; 11 October 2025
2.4-mile swim/112-mile bike/26.2-mile run
It’s the Big One on the Big Island of Kona – triathlon’s most historic, iconic and hallowed destination where the greatest elite and amateur athletes descend each year to tackle the sport’s own beauty and the beast.
The Ironman World Championships might not be the hilliest or the highest course, but what the stats don’t show is the searing heat, stifling humidity and fearsome crosswinds do to even the most experienced of competitors. But complete the 2.4-mile (3.8km) sea swim, 112-mile (180km) bike and 26.2-mile (42.2km) run and you’ll forever be a legend of the sport.
Escape from Alcatraz, USA
San Francisco, USA; May 31 - 1 June 2025
2.4km swim/29km bike/12.8km run
Like Norseman, Escape from Alcatraz commences with a leap from a boat – surely the greatest way to start a triathlon – but this time from outside the former-prison island of Alcatraz in San Francisco. Each year sees over 2,000 athletes make the jump from the San Francisco Belle steamboat into the bracing waters of San Francisco Bay for a 1.5-mile (2.4km) swim to shore. The Golden Gate Bridge looms over the 18-mile (29km) bike leg, before an eight-mile (12.8km) run finds racers hit the sandy trails of the Golden Gate Recreation Area and its formidable Sand Ladder – 400 near vertical steps up a cliff face – before the finale to make this, alongside Helvellyn, one of the world’s toughest ‘short-course’ triathlons.
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