i just spar like crazy personally but thats because i despise running and have no intention of becoming a professional fighter.
Spar, spar, spar some more and your fighting cardio goes way up. Esp if you spar with some fresh guys when u get tired.
Running is def good, too bad im flat footed and hate it with a passion.
on the flipside, i know guys that could outrun me all day and not last 3 min rolling with me, its just a diff kind of cardio.