How did the speaker feel when he ran his first mile? He felt it was the ______ t

My favorite sport is running. But when I ran my first mile, [16]it was the hardest thing I have ever done in my life. Now [17]I have been running for 15 years. [18]Each time I cover as much as 8 miles, and I have 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. [19]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. [20]Running is also something you can do at any time, during a lunch hour, or in the early morning.