To Improve or Not to Improve

I'm a junior in high school, and this is my third year running cross country. My goal for this year has been the same as my goal in my prior two years of running, which is to improve my time for the 5K race. This involves doing a lot of speed work, which helps make me faster, but also is unpleasant. For example, this past week I had a workout where I did a ladder going from 0.25 miles to 0.50 miles to 0.75 miles to 1 mile of very fast running, and then went back down the ladder. This was very difficult, but I worked really hard because I knew I wanted to improve. By the end of the workout, I was very tired, but was felt confident that my time would drop at least 30 seconds because of the hard work I put in. To me, lots of things in life are hard and/or painful. If you really want to accomplish something, then the only way to make it happen is to put good work into what you are doing.