I just bought this game pre-owned the other day (of course the person before me took out the TBL Club Membership code
) but I finally got everything set up and I did my first 30 minutes today on moderate even though it suggested challenging.
I liked it a lot. The menus were a little hard to navigate at times because my little ball of light kept jumping around and sometimes Bob couldn't hear me tell him that I was feeling good and didn't need a water break, but other than that I enjoyed it. I never felt like it was too much or too little. Like you, I also have knee issues so I can't do the lunges very well. Still, compared to the Nike+ Kinect Training game this one feels like a dream because I'm sweating but not feeling like I got hit by a Mac truck afterwards.
I don't know what your goals are, but I'd like to keep in touch and see how you are liking this game as you move through it. I set up my goal for a 12 week program at 3 days a week, but I'll also be using up the last bit of my gym membership during that time.