Many of us have had similar experiences... maybe the health concerns are a little different and the circumstance... but the same.
No easy answer.
It is difficult to see parents age. The clock is different for all of us.
If you believe there is an illness... then encourage treatment. Same with healthy living.
Just tell her openly about your concern in a non pushy way and let it rest (assuming there is no immediate emergency).
You father is probably the most influential person in her life. If he cannot influence her.. then perhaps her doctor. But, don't you think she knows what it means?
There is a very simple non-health club activity your mother and father both can do daily together... get out for a walk. See if you can get your father to take charge of organizing that daily activity and ensure it happens... if they can do it... maybe a mile or so a day. If you think your mother and father may have problems with that type of activity, perhaps they should visit their doctor just to make sure it is ok.
Don't let your concern (or a debate) spoil the visit. Enjoy your time with your parents.
You did not describe your situation (if you work, don't work, etc)... but you should probably plan some visits back home with your parents if you can.
Here is a bit of, perhaps depressing, but real insight... you are beginning to be aware of their mortality... But don't grieve prematurely. Their vital years are limited but they are here and this is now... Enjoy your time with them!