To be wildly pedantic, an ultramarathon is anything over 42.2km (26.2mi). It's perfectly possible to do an ultra with an Apple Watch if you're only going 50km (

To be wildly pedantic, an ultramarathon is anything over 42.2km (26.2mi). It's perfectly possible to do an ultra with an Apple Watch if you're only going 50km (even I can manage 50km within the 8h battery life.)