I have a suggestion for you. I am not part of this but I have been doing some cardio for about an hour a day (30 minutes in the morning 30 in the evening). What I did was go out a buy a Elliptical trainer. It is a cross between running and crosscountry skiing. Your feet never leave the pedals so there is no jarring to the knee at all it is very smooth and yet you can get up to the speeds that you are talking about. You can find them in any gym as well probably in the same vicinity as the treadmills.