Running is my centering activity. If I do not run, my mental stress cache is not cleared. No other form of cardio quite does it for me. Swimming, cycling...neither meets the two essential components of running that I require: intense cardio and movement through the world. Swimming is intense. I can potentially get a better overall workout after a long swim over a long run. But I'm monotonously going back and forth in swim lanes. Running allows me to focus on my breathing and my surroundings. My mind is singularly on those things. While I'm swimming, my mind can still be spinning. Cycling gets me out in nature, but it's not often the intense cardio circuit that I need. So, running is the best, easiest way to access these two needs.
I've been running for a long time. I didn't do track and field in high school, nor cross country (though I did one day of try outs). I ran by myself, for my own needs. I think this has helped me sustain it as a practice. It was never out of competition. It was because I felt good afterwards.