I had a jack russell diagnosed with luxating patella at 12 months old.

We decided against surgery and she was put on supplements and kept very fit and active.
She lived til she was 16 and during that time was used on shoots flushing pheasants working several hours four or five times a week.
She also went rabbiting and ratting running miles daily.
Her patella never stopped her living a very active life,infact the vet did say that it was her lifestyle that kept the arthritis minimal and she only showed a stiffness in the last year of her life.

I hope this eases your worrying a bit xxxx