This is a truly random and totally abstract question: do you need a cycle to take part in an audax or could you, theoretically, do it on foot? IanN calculated yesterday (yes, I shared this stupid thought with him) that if an Olympic class marathonist could carry on at the same pace for another 160km they could complete a 200 within the time limits. Someone, somewhere, is bound to be able to do it – but anyone who can do it surely has their sights on bigger goals than a somewhat obscure, slightly geeky and non-remunerative semi-sporting activity! (I reckon the answer is that AUK regs don't actually require a cycle, just "human muscular effort", so it would be allowable.)