In my opinion I think running is essential for any fighter,years ago I always trained/sparred with pro boxers and the ones that had'nt been doing their roadwork always got exposed when we sparred in the gym they either got dropped with body shots or were blowing after a few rounds!
I agree with what's been said earlier doing hill runs/interval runs/sprints,etc they all build up that explosive power needed for fights and give you that much needed edge!
I preferred doing a long run once a week and shorter runs of 3-4 miles maybe 4 or 5 times a week but it's down to what your own body can take as well and you have to listen to it.
When I used to fight my own training was split with early morning runs up at 5am in the morning then go to work do an 8 hour shift and back to a two hour workout again at night.
Did'nt get chased by as many dogs early in the morning too!...lol