hardest根据“...it was the hardest thing I have ever done in my life．”可知应填hardest。

My favorite sport is running. But when I ran my first mile, it was the hardest thing I have done in my life. Now I’ve been running for 15 years. Each time I cover as much as 8 miles, and I’ve developed a habit of running everyday. Many people like running. There are several reasons why running seems to be the best form of exercise for them. For instance, it is something everybody knows how to do already. So no special training is needed for it. Secondly, it is something you can do anywhere. Of course you must have a good pair of shoes. You can run if you’re leaving the city. You can run when you travel. Running is also something you can do at any time, during a lunch hour, or in the early morning.