Last June I started running with a group of experienced runners with an impressive list of races and race times between them all. When I asked them what they did to cross train, more often than not I heard SPIN.
I had taken a spin class at my gym a year earlier and found it to be very challenging. I remember only being able to do about half of what the others were doing, and definitely at much less resistance. Afterwards, I could barely walk and my bum was sore for days. I thought I was in pretty good shape but this class really proved me wrong. I didn't go back to that class.
Needless to say, hearing that everyone else was spinning to cross train made me decide to give it another try. I've only been able to do it once (and on a few rare occasions, twice!) a week but have been taking a spin class since mid-October. And I absolutely love it. There are incredible benefits for a runner. See this article.
If you haven't tried spinning, I highly recommend you try it. It's great on cross training days and also a killer workout when you have time to do both run and spin. I call it the Double Whammy and I absolutely love when I can do one of those workouts!