I think running is good cross-training for cyclists. I started running about a month ago, took it slow, and can already tell a difference in my breathing.

I say go for it, just start easy. If your body isn't used to running, it takes a long time for your joints, etc. to get used to it. I'm just now feeling comfortable as I run. My DH was an elite college XC runner and has been helping me get started. I'm always surprised at how small the weekly increases are. I feel like I could go farther, but he says it's best to take it in small tenths, etc.

But I still stink at hills...must go practice more...