Ha! spent a while writing, then got distracted. 20+ replies since I started!
I know what you mean. There are other sports which came to mind, though.
Surfing in particular: there’s something special about paddling out when it’s big and scary with lots of water moving around. Analysis of the currents and the sets to pick a time and a place to paddle out. Anticipation of what a waves going to do ahead of you, timing and technique with your duck diving to minimise the drubbings, and the balls-out determination of paddling past the edge of lactic burn, interspersed with breath-holding and the odd kicking knowing that that it might take 10, 20, 30 minutes or longer to get out (if you do), and that slacking for a few strokes might be the difference between sneaking out back and getting caught by a big set, taking a beating, being washed in and starting all over again.
And if you make it out, constant paddling to stay in position, dodging big sets, being in the perfect spot for the best wave you’ve seen so far but without enough puff to catch it. And if you’re lucky/ballsy enough, maybe some actual surfing. I don’t think I’d be alone in saying I’ve had a few zero-wave sessions when it’s been big.
Ski and splitboard touring on challenging terrain I’d say is pretty similar to mountain biking in terms of all over body and mental workout – even lower mileage though, I’d struggle to ski-tour the SDW in 12 hours!) . Long alpine climbs and ocean yacht racing must be pretty tough too.
14 hours of circuit training would probably do it too!