My own experience may be useful here.
I was diagnosed by a consultant as having osteoarthritis in my left knee. After both losing weight and just kind of putting up with the remaining pain for several years (the medical advice), a friend suggested that I see a physiotherapist.
I did, and the physio could find no sign of osteoarthritis; it turned out that I didn't have it at all! Instead I had weak quad muscles which meant that my kneecap was squint when moving over my kneecap. Which is painful.
The physio proved it by giving me some simple thigh exercises which within a week or two made the pain go away entirely.
I'm sure some unlucky people *do* get osteoarthritis; but my advice is to be doubly sure and check with a physiotherapist rather than just believing what GPs and hospital consultants tell you. In my case they didn't have a clue, and (very confidently) got it utterly wrong, leaving me in pain for several years.
I saw a physio at FASIC for a bit under £40, at very short notice too, but no doubt there are other good ones.
My own knee problem simply made me use progressively lower gears over the years for going uphill.
Losing weight also helped me a lot.
Finally, for what it's worth, the physio told me that cycling isn't the most complete exercise for your thighs as you use the muscle only in flexion and not in extension (I think...) - she recommended briefly stretching my quadriceps after every cycle ride to make up for it.