I'm 72 and have never had a knee replacement but from what you're saying I'd suggest going to your local bike shop and get fitted to a new bike. Your body may have changed, your knees may want something you never thought of and the shop owner can get you on the right bike.
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The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.