Your mission to Mars: the answers
After 220 days, the changes in fitness are shown above.
After six months of low gravity on the surface of Mars followed by 220 days of zero gravity on the return journey it looks like the astronauts would be in very poor condition by the time they returned. New rocket technology might reduce the transit time to below 50 days, but such technology is still a long way off.
The reduction in fitness over time is described as linear. This is probably unrealistic and is likely to level off at some point. What is required is a more complex non-linear model incorporating more parameters. At present, without data from several long-term space missions it is difficult to construct such a model. There are also likely to be synergies (compound effects) with other medical issues not discussed here, such as changes in blood composition or hormonal variation.