Mate, it's common sense that you have to be fit to be a fighter, be that boxing, kickboxing, mma, etc.
Still, fighting/sparring is the bread and butter of it all.
Look at Chwis Eubank, hardly did any roadwork at all.
Chavez, didn't learn any of his tricks doing fancy pad work or any of that, learnt it all sparring.
Read the interview with Manny Steward, said he learnt loads from Chavez and he was one of the best ever.