I agree with Sarah. You just cannot find something to completely substitute for running. Plain and simple. You can easily find Cathe cardio to compiment. Personally, I try to do 1 IMax each week, about mid-week. If there is a day that I simply do not FEEL like running (for whatever reason) I will pop in an all-step workout that is steady-state cardio...my favorites are Rhythmic Step and Step Blast. I make every single move hi intensity. Then I'll return to my usual running on my next scheduled run. Not usually a problem.
In fact, after my Janaury marathon, I took 2 full complete weeks off of everything. On the third week, all my cardio consisted of all-step workouts. On the 4th week, I added running back in and was easily able to put in 6 miles.
I hope this helps.
Also, like Sarah (and thanks to people like Sarah and Carole) I have been adding all Cathe weight work around my running, my favorites for lower body are PLB and L&G also.
Gayle