I am traveling with my single entry Schengen visa, 68 days duration, to Greece for 40 days and to visit Paris for 5 days. Will that be okay?
A single entry Schengen visa means you can only enter the Schengen area once. Both Greece and France are in the Schengen area, so you can go from one to the other without leaving the area (and so without needing to re-enter it).
If you attempt to go from Greece to France via someplace outside the Schengen area, or leave it for some other reason, you will not be able to return.