When a lifelong friend and former member of SEAL Team 6 went through BUD/S, he said the first thing they did was go for a 5 mile run. At the end of the run, some guys were falling out, panting on the ground. When the last had come in, the instructor said, "That was fun, let's do it again!" A bunch of guys couldn't.
The lesson was to never decide it's over until it's over. It's something that stood me well (and I never was in the service) in the oil patch, because the idea that you had to only go 'so long' was always subject to change. One job ran 180 days straight. Others ran 60 or 70 or more. When the kids would ask when I would be home, they learned the answer would be "I love you, I miss you, too, but not until we are done."
I'm hoping the astronauts can adopt that attitude for their mental health, and keep up with whatever exercises and diet they need to to stay healthy.