Wheeze is entirely correct in his decision, there comes a time (most likely age related) when the body does not function as it did and damage is being done. Irrelevant of how motivated we may still be to race or run the hills only a fool would continue to ignore the warning signs (pain is a good one) and carry on regardless. Probably later seeking sympathy when surgery for "new bits" are required.
Giving up fell running is not the end of the world, you simply have to mentally adapt to the next best thing that your body can manage. Walking, biking, swimming or whatever, I turn 70 this year and am damn thankful I can still do what I can (currently nursing bad Achilles, so bike it is) when I look about the country at others of my age.
Keep plodding on, but accept reality when it stares you in the face.