1. Injuries--knees, back, etc. Your fitness doesn't matter when your body doesn't function. Running at 50 for many is like Cole Hocker running with a bad sprained ankle. Fitness cannot overcome functionality.
2. Not having any desire to try to run fast in part b/c you will never come close to your former times.
3. This said, I think former runners have a core fitness that can be re-activated. Some guy who ran 14:00 for 5k in 1995 and now runs 26 minutes at Turkey Trots and runs a few times a week at 9:00 pace probably could get down to 17:00 in a few months if he wanted to (but, see point 2, why would you?)
