I'm 46, and have one knee which an orthopedist has diagnosed as "arthritic."

I commute 28 miles a day. Lately, I've been riding mostly fixed.

I like about 70". I use SPD clipless.

My number one suggestion would be to put a brake on your bike. If I get any knee pain at all, it's usually after I've been stopping without my brakes. Also, trackstand sparingly.

The funny thing was that I rode my 27 speed Surly Crosscheck two days this week. It could be a coincidence, but I actually got a little bit of knee pain for the first time in a while. I went back to fixed gear on Friday, and that minor pain has gone away.