Your mind and body are not separate. They are connected.
In fact in the process of training one, you must also train the other.
Whether you choose to be aware of this or not is up to you; many people go into the gym just to train their bodies, without thinking much at all about training their minds while exercising.
They will often make a lot of progress – but only to a certain point.
It is only when you combine the training of the two, when you push your mind to its limits while your body is simultaneously being tested, that you will achieve your true potential, both physically and mentally.