Just to throw in my 2-cents . . . I run 12 - 15 miles a week, broken up into 3 mile hills and flat 4 - 5 mile runs with intermittant sprints.  Personally, I believe running is quite important in my training.  Along with a little weight training, the two can provide a fighter with a strong heart to keep energy rich blood flowing to the extremities.  I've found this combination to translate into much needed stamina and strength during spar sessions.