I've been doing Crossfit for a few weeks now, and I've definitely noticed improvements in my run timings (recently shaved half a minute off the timing of a 4k run a year ago after adding more than 10kg on my frame). But in my opinion, I don't think Crossfit is in any way superior to a well thought out and executed running plan, particularly at times when you have to stop to rest before attempting your final set of 30 snatches when your legs could probably have kept going during a run.
That said, I suppose Crossfit has probably got a good carryover to races 5k and shorter i.e. the typical duration of WODs. Anyway, if you really like Crossfit, a good idea would be probably to incorporate both Crossfit as well as longer distance running (e.g. 30-45mins) for an endurance base since Crossfit already provides the HIIT effect.