“Jack of all trades; master of none.”
If you wish to compete at a high level at any particular sport, you have to realize the importance of this quote. You must practice as specifically to that chosen sport as possible. If you are competing in marathons, the majority of your training regiment should consist of running. Intelligent choices in cross training will definitely improve your success in any sport, however, be careful not to overdo cross training. Time is limited so we should leverage smart training versus bad training. For example, BJJ is a sport that relies on your reflex, timing, technique toolbox, muscle memory, and an ability to control adrenaline. Strength is only one aspect that may help for achieving gold medals. If you lift weights all the time you become great at lifting weights and building muscle, but someone who dedicates their life to training jiu jitsu will usually submit you at will.