ATTQOTD: honestly, i'm pretty much ready to handle whatever surface a given run/race throws at me. i've had issues with concrete and extended distance training but that's an outlier. what i will advocate is occasional trail runs. i was having a number of knee issues a few years back and a fellow runner suggested trail running as a possible solution. the logic being, you are forced to use almost all the muscles in your lower leg due to the irregular running surface (and, hoo boy, i can tell you by how sore everything was from my calves down, that's the truth). this means better support for your knee and, possibly, less problems from the joint. it worked for me and i found a good trail run twice a month was a really good variety to my running regimen.