This change in focus really improved my approach to running years ago.
Instead of making my runs a matter of distance, I made them a product of intensity. I'm more likely to run "an hour" than run "5 miles." As my performance decreased with my age, I found that the experience of running had not changed. The secret is to focus on logging in the time rather than the same number of miles. This helps me to maintain my endurance while never approaching fatigue. I train about 5 hours each week at a slow speed. I then use weekend races as my high-intensity training.
"Think quality, not quantity" has become my new mantra. This is simply returning to the first rule of exercise: Listen to your body.
If you're a runner, hope this helps!