If you are a sportsman, be it professional or amateur, fitness - physical and mental - is of utmost importance. Here's a very pertinent quote from Paddy Upton, physical and mental conditioning coach of the Indian cricket team.
Fitness is an attitude. If you're training because somebody tells you to train, it's difficult. But if you're training because it's the right thing to do and you want to be disciplined, then it’s easier. Fitness is everything, from having energy to remaining injury-free, to being able to perform in an atmosphere of back-to-back one-day internationals... Some of the biggest mental challenges in a sport are getting injured, getting dropped, and when you have a low run of form. That’s when the individual’s character is really tested. There's a natural process you go through of disappointment and frustration, sometimes even wanting to give up.