UK version
Endurance sports, triathlon, cycling, running.
Ride across Britain
Probably takes less effort from all three.
Ride across Britain by bicycle from top to bottom (or other way around).
Famous John o’Groats to Land’s End route is a must do for every cyclist who resides in UK. And it’s not so hard as it sounds, 875 miles (1400+ km) omg! If you spend summer cycling and doing few 100 km rides at least few weekends, you’ll be ready for it in autumn.
Solo ride might give you more pressure and add some extra weight to your bike but there’re options from official events you can take part in that also will provide you technical support, one of them is Deloitte Ride Across Britain Event takes place in September.
Solo or as an official event — choice is yours. 14 days or 9 days — choice is yours. One thing I recommend — do it in first half of autumn — it’s the best time for this adventure if you don’t want to overheat. Don’t forget that you’ll be rewarded with the beautiful scenery along the way.
Bonus: London to Paris as warm up.
Swim across English Channel
This might sound crazy for non-swimmer or triathlete whose swimming discipline is the last he picked up.
Something like this requires preparation in open water and sea swimming, and somebody who will provide you a guide for your training. 21 miles (32 km) is approximate distance crossing English Channel according to CSA website and conditions are far from usual morning swims in your local swimming pool. Still, nothing is hard if you train and prepare for it. I recommend you to visit and research Channel Swimming Association website.
According to website, 2373 swims done to date.
Bonus: Loch Ness swim.
Do Marathon/Ultra Marathon in unusual conditions
Finally, running.
Everyday runs can become boring if you train for one type of event again and again. That’s why you need to diverse your targets.
UK conditions are boring, maybe you could go to Scotland or Wales and do some trail runs but come on — these are usual conditions and nothing special about that.
One of the exciting running events takes place 4000+ miles from the UK— Baikal Ice Marathon in Siberia, Russia. People from 50+ different countries travel thousands of miles just to run in freezing conditions across frozen lake Baikal.
Highlighted by several publications, including Express and Men’s Health, Baikal Ice Marathon takes place in March 2019. Head over the website for registration info.
Bonus: Snowdonia Trail Marathon / Ultra Marathon.
Just remember — everything is easy when you prepare for it.