I've heard in a few places that you shouldn't do too much interval training. Twice per week seemed to be a good recommendation. So if you run and cycle, should you limit interval training to once per week for each sport? I'm guessing the point is to give your cardiovascular system enough time to recover.

I don't think I have the willpower to do more than two interval days per week, so I'm just mostly curious if it would be a bad idea.