I can run very fast for long
Really? Long-distance running, or endurance running, is a form of continuous running over distances 
I can perform in both. I use to be a marathon runner
Marathon
The marathon is a long-distance race with an official distance of 42.195 kilometres (approximately 26 miles 385 yards), usually run as a road race. The event was instituted in commemoration of the fabled run of the Greek soldier Pheidippides, a messenger from the Battle of Marathon to Athens, who reported the victory. The marathon can be completed by running or with a run/walk strategy. There are also wheelchair divisions.
The marathon was one of the original modern Olympic events in 1896, though the
Multi-day race
Multiday races are ultramarathon running events which are typically either segmented into daily events of a specified distance or time, or staged so that runners can run as far as they want, at their own discretion, over a set course or over a set number of days. Multiday races can range from continuous 48-hour track events to staged transcontinental treks.
Beyond the marathon.
Very long endurance running events can be divided into three broad categories: the traditional marathon, the ultramarathon, defined as any event