Answer:
Saying that relationships need "exercise' to survive means even relationships need to be taken care of, they need work to be maintained or even get better.
Explanation:
Just as an individual needs to have proper exercise to keep fit and healthy, so also a relationship has to undergo a sort of 'exercise' to be functioning properly. The 'exercise' here doesn't necessarily mean the literal working out, running or doing any physical body movement. 'Exercise' could also mean a constant or often attempts by the people involved to make things better or change the bad, so that the relationship will flourish.
For things to get better and improve, just staying dormant and doing nothing extra isn't the way. Likewise, even relationships need hard work. They need to be given attention, changes wherever needed, and even sometimes forceful application of things. One needs to put efforts in it, so that it will not fizzle or die away.
