I ran an 18 minute 3 mile PFT.... ONCE. I never ran it slower than 20 minutes. I've always had a little trouble with my left knee, but when I tried to run a few weeks ago I just couldn't do it and the next day both ankles, knees, hips and back were all hurting so bad I could barely get out of bed. Part of this is due to my body just not being used to the impact anymore, some of it is because I'm carrying more weight, 170 when I was 18, 225-230 now.
I'm only 26, I turn 27 in a few days. I should still be in my prime, but I look, feel and drive like an old geezer. Had a bartender guess my age at 43 when I was 22. And when I drive the wife's Caddy on the interstate, I go 71... in a 70 zone.