There is a certain phenomenon that happens for many runners that makes running way more than just exercise or a competition, and honestly it's the primary benefit I personally get from running... it's the spiritual experience that comes from your mind and body working together, forming an addictive union that's a bit difficult to fully explain. It's not the "runners high" everyone talks about, it's something a bit deeper and more powerful than that. You can't really deny something you experience firsthand, but it's certainly open for interpretation, so not everyone who has felt this will come to the same conclusions about it as I do.
I've been a Christian for about 10 years, and had ups and downs over that period of time, even stretches where I tried to walk away from God, but you can't abandon something when you really do believe it to be true. I respect other beliefs, I even find great truths and values in some other faiths, but when it comes down to it, I see it all pointing towards Christ. I'm not explaining my faith to convert the people that might read this or to argue over religion, but to more effectively explain how I interpret my firsthand experiences. No matter what specific conclusions you come to about what you believe, spiritual experiences are very real events, and I believe running is one means by which they occur for me.
In "Chariots of Fire", probably the most famous movie concerning running, one of the main characters explains his motivation for running with the oft quoted line, "I believe God made me for a purpose, but He also made me fast, and when I run I feel His pleasure." Whoever wrote that line in the script is almost certainly a runner and has a pretty solid understanding of worship. That union I talked about before is personally my purest form of giving praise to God, so when I first heard this I instantly related. God, to my dismay, didn't make me very fast. He did however give me a talent to run far and a passion to love doing it.
When it comes down to it, I think every relationship and every experience reflects a portion of God's character and His plan for us, and for me to embrace that recognition and fully enjoy it is worship. My feet striking the ground in appropriate rhythm, my breathing and heart rate evening out to the place where I feel like I could run for days, my back straight, head up, and feeling each muscle in my body almost individually. Then reaching the point where my head clears... I don't have to remind myself about form, the elements aren't concerning even if they are inclement, I'm not nervous, I'm not angry, I'm not sad, or any of the other negative emotions I almost certainly felt at some point in the day before my run. I stop looking at my watch, I stop worrying about where I have to be later, or worrying about a cut off time... I'm just running. That's exactly where it happens. Your mind, perfectly clear a minute ago, floods like a dam was just breached... you begin to notice everything. It starts externally... the trees, the trail, the sky, the breeze, the rain, the snow. Then you begin to think of how grateful you are to be able to experience all of it in that moment. This is where you see people fall in love with running. This is where people like me become ultrarunners. This is where I experience God. It's a spiritual experience for some, but for me it is the unadulterated worship of my Creator.
I don't think it's any coincidence that the Apostle Paul compared following God to running a race, it was very deliberate and he did on multiple occasions throughout his letters. The ups and downs, the pain, the elation, the camaraderie, the importance of training and preparation, maintaining yourself, being aware of the course, having patience, working through injury, they are all perfect analogies. The longer the race the more evident the parallels are. The absolute joy and relief in finishing, the desire to see others cross the finish line, and then celebrating with each other? That's Heaven.