I've just been reading an article about the Paris-Brest-Paris Audax cycling event: it's 1200km, with a cut-off time of 90 hours, and the record is about 42 hours. So comparable to one of the longer ultra running events, you would think.

However, 6,500 people started this year's PBP, with a 69% completion rate; which suggest to me that it is significantly less demanding than a running event taking a similar length of time. OK, so an Audax is not principally intended to be a race. Yet cycling long distances does still require physical strength and fitness, and mental strength, similar to running.

So is the difference simply down to cycling being a low-impact activity? For the same expenditure of energy, does running cause greater fatigue because your feet are repeatedly slamming the ground, rather than moving round in smooth circles as on a bike?