I've had some knee issues since last month so I've been trying to take it easy. Had a couple of rides where it started hurting about 20 miles in, then made it 26 miles last week without a problem. We did 27 miles on the tandem yesterday, pretty much all flat, average speed 16.6 mph; no knee pain. Today I did 34 miles, 3000 feet of climbing, average speed 15 mph (crazy headwinds the whole way, both directions, I'm not kidding); knee is still okay. Seems to be rehabbing okay, but I am still going to my sports medicine appointment this week to find out what is going on and hopefully keep this from happening again.