My sports teacher used to let me know when there were events on and I would just run in them for fun. Ez és az alábbi mondatok jelentése? Mit jelent az "event on" és az "run in"?

1.I love preparing myself for races which are very demanding?(itt a demanding mire vonatkozik?)

2.And it's very satisfying to focus on something so specific.

3.It was amazing to find that what I had achieved meant so much not just to me but to so many other people.