People ride 1200k in 90 hours and less all the time without knee pain. It's not quantity, it's doing it wrong. If you do it right, you ride across the US in 30 days - or 9. In my mid-60's I rode 400k in 15 hours, and I'm not a talented rider, just efficient.

I had an episode of knee pain when I was just picking riding back up at 50. As soon as I figured out how to do it right, it went away and never returned.