Best wishes for successful outcomes from any and all surgeries you need to correct the knee issues. I have an acquaintance that has ran 6 or 7 half marathons since his knee replacements. I'm facing issues myself except in my hips that are keeping me from enjoying running as I once did. I thought my half marathon last spring would be my last buy I have stubbornly started training for another in November. Fortunately the hip doesn't seem to hurt during riding.
I am looking for a mixte just to have ready if and when the time comes that I need to change bikes due to the inevitable effects of aging. I hope I can find a Takara mixte because like you, I really like mine as well.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke