I haven't done a DU (yet) but I went down and watched one last month so I could see what people do wear. Almost everyone was wearing cycling shorts and tri tops or tank tops. I saw 4 people wearing regular running shorts but they were either very young, or looked "recreational" (not serious bikes, no cycling shoes etc). I have run in cycling shorts and they felt ok. My du is only 5km run before and after a 32km ride.