It could be the bike fit that is causing your pain, it might be a good idea to give some more information about where in your knee the pain is occurring and your particular bike. Also, post this question and the additional information in the bike fitting forum. The saddle position and height can cause knee aches and trying to push high gears rather than spinning the cranks can make the knee(s) hurt. A combination of saddle height and pushing big gears did this to me in the late 70's, I went a bit lower on the saddle and began spinning rather than grinding out a big inch gear. Also, if you are using cleated shoes and clipless pedals the position of the cleats can cause knee pain. This requires you to adjust the cleat(s) to suit your feet and legs correctly.
Best of luck on solving this problem, don't let it stop your riding it is most likely a simple adjustment. And, welcome to the 50+ Forum and to Bike Forums.
Bill
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Semper Fi, USMC, 1975-1977
I Can Do All Things Through Him, Who Gives Me Strength. Philippians 4:13