I've run for 45 years and can honestly say that running has never NOT been fun for me. I've never felt bored, stale or burned out. Maybe it's because I simply love the feel of the run. I don't "do cardio" , I just go out (never indoors on a treadmill) and run. I love races and slow meandering runs. Roads, sidewalks, trails or tracks. They're all ok. Maybe it's that I seperate running from the concept of exercise. Instead of thinking that I HAVE to run, I take joy in the fact that I GET to run. Always have, always will.