I am 72 years medium. Not young or old just medium. Had quadruple by pass surgery 3 years 4 months ago. Have been running since 19. Throughly enjoy my running even if I have slowed down. For some reason I started on a streak 187 days ago and during this time I have ran 939 miles. I have been fortunate to not having a real running related injury over the years. Knees are fantastic. Recovery times takes a little longer. I do all my long runs at average heart rate of 125 bpm. Only time I really feel depressed with my running is when I am passed by some one with a walker. Truly I love running my 11 -12 minute miles don't bother me. And truly feel elated when I run under 10 minute pace for my 5k. Ageing for me has been a wonderful time and thankful to still be able to run.