When I run, I enjoy it. If I didn't enjoy it, I wouldn't be running. I also enjoy being fast and it works out really well when you race. However, I raced that 50 mile race, the enjoyment turned into survival. What can I do when I get to that point? It's basically like when you're 30 minutes from being done of a long day of work... and it takes so long! Completely different than watching an action movie and realizing after you get out of the movie theater that a couple hours have already passed. Time seems to fly when you're having fun, but it slows down considerably when you aren't.
It's all about the state of mind you're in. If you are enjoying yourself, then it just flies and when you're done, you don't know what hit you. If you aren't enjoying yourself, you focus on the wrong things and forget about the right things and time seems to move slowly. Does time really change speed? No. If it did, we'd be living in a messed up world. The reason we let time fly by is because we are enjoying the moment and focusing on the fun. The reason we let time slow down is because we focus too much on it and our impatience works against us.
Today, I spent 9 hours on my feet working as a runner for an outdoor event, serving food and making sure everyone had the right foods. It was tough... but it was a lot of fun. I enjoyed going from person to person and just spending time with people and getting to know them as I gave them their meals. By the end of the shift, time slowed down as the crowd thinned, and as it came to a close and I went to sit down in the car for the drive back... I found out how tired out my legs were from standing and walking around all day. It was pretty exhausted, feeling so~ good to just sit.
We control our bodies so much that we can actually 'forget' how tired we are. This art of forgetfulness is only a part of what it means to have your mind over matter, but is very important. Sometimes, in a race, you come to a point where you want to give up. The only thing pushing you is yourself, and it's up to you to continue that path forward and grow.