[nsndnd]

What's the length of the run?

50k could refer to 50km, yet, ultra usually refers to longer runs like 100km - 50 miles are 80km, so what would make more sense.

Judging by the time per mile they state in the article, it's 50km.

[have_faith]

Ultra refers to runs longer than a conventional marathon (42km~). 50km is generally the starting point for the ultra distance though and 100km is a more common benchmark.