This gradual build-up is reminiscent of the tale of ‘the farm boy and the calf’. As the story goes: on the day a new calf was born, the young farm boy picked up the calf and carried it all the way around the barn ten times. He did this five times a day until the calf grew so big he could no longer lift it. By that time, the boy could lift over 135 kilograms (nearly 300 lbs.).
The crewmembers will only need to lift their own weight to move around freely, but the concept is the same. Little-by-little their gravitational weight (centrifugal force) will increase and, presumably, their physical strength. In theory, they could come home from Mars in as good of shape as when they left Earth.