Lesly,
I too needed a sub for running. As Traci mentioned, any steady state cardio will do. I find that any of Cathe's step DVDs are perfect.
I recently bought her compilation Rhythmic Step/Imax/Maximum Intensity Cardio DVD and boy, are these a killer. I did all three for the first time (on different days of course) and had a terrific cardio workout that is comparable to running.
HTH!!
Eileen