Oh yes, back to your 20+ miles. Congrats. As a sprinter, I pretty much died in my tri after 10 miles.
As for my knees, it's definitely not the "burn" you get from working hard. It's more like "oh, the medial part of my knee feels like it's being stabbed when I go up steps" kind of feel after I ride. It happens sometimes when I lift too. Probably not so good.