I have been doing the P90X at a slightly slower pace than they suggest, but am still seeing huge gains in muscle. Unfortunately, I also have bruised knees. I want to do more push-ups, but can't if I don't do them on my knees.
Every time I look at my knees, it's a reminder to keep working hard. It's also a reminder of all the hard work that I have already done. I kinda view them as my badge of honor.