Yeah, in the U.S. that temperature would be considered fine although not perfect (16C with a light rain is considered better--for the runners, if not the spectators). But 21C is definitely in the "comfortably cool" temperature zone. I don't think I've heard of anyone dying of heat stroke at any of the local marathons, although our weather is so unpredictable it's not unusual to run them in 25-30C weather, but there has been a case of death from water intoxication.