Fitness is 'task-specific'

Fitness is 'task-specific', meaning the nature of how we spend energy during exercise depends on what we are doing.  As we become more and more familiar or conditioned to a task, we spend less energy achieving specific output levels.
The job of trying to become 'fitter' divides into two phases: An early phase we can call the strengthening phase, and a second or later phase we can call the work capacity phase.  The purpose of the first, strengthening, phase is to ensure we use the correct muscles, and time is needed also to ensure these tissues are appropriately adapted.  The strengthening phase of fitness can take many months or years because the brain must first figure out what you're asking it to achieve and which muscles are best suited for the task.  As we age, this process takes longer because the 'motor program' or instruction manual depends on the quality of the information that flows from the body to the brain during exercise and inactivity, characteristic of adulthood, leads to a poor flow of information.
The second phase of raising fitness, the capacity phase, overlaps with the first phase in the general sense that all repetition and practice improves capacity but care must be taken to ensure the body is structurally sound and movement is efficient before significant load is imposed.  Mike Hosking was on the radio yesterday complaining of recurrent leg muscle strains when he runs.  The basic reason he is experiencing these strains is an incomplete strengthening phase in his running.  He possesses structural faults and/ or faults in the way he applies force when he runs.  The moral of his story is to seek help when it is obvious something is wrong.
Jeremy, Aug 2017