In evolution, fitness1 is about success at surviving and reproducing, not about exercise and strength. Of course, fitness is a relative thing. Reproductive fitness2 reflects the ability of individuals to pass on their genes to subsequent generations.
Fitness traits, also referred to as life-history traits, include measures of fertility and mortality and are complex phenotypes that are direct targets of Darwinian selection.
A genotype’s fitness depends on the environment in which the organism lives.