The answer really depends on what you're goals are. What I mean is, if you want to run races, then you may choose cross-training that uses similar muscles... I don't see that as a bad thing, in part because I'm not going as hard as I do when I run. This still allows recovery. Cycling use muscles in a somewhat different way and IMPORTANTLY to me, do not take the same pounding that running gives. Cycling is one of my chosen cross-training activities (for me it's because it strengthens tendons without the same explosive impact that running has). I also like Yoga, which also exercises my legs (and strengthen's upper body without bulking it up too much). If your goals do not include racing, then cross-training could be anything, such as swimming, which builds more upper body strength. Because I do have racing goals, I will swim, but it will not be a focus of mine... I will even periodically weight train, but higher reps and it will not be a solid focus... I'll do more yoga and bicycling.

Mark