Thanks for the info about rim size and comfort, I didn't see that in
any of my research so far.

So the Giant FCR3 (700cc), Giant Sedona (26"), Cannondale Comfort 5
(26") and the Redline D440 29er (29") would all be good bikes for
exercising. The main difference in comfort would start at their wheel
diameter, and then if I could use a mountain bike sized tire on it. So
the either the Giant FCR3 or the Redline D440 would be the way to,
based on wheel size alone. Now all I have to figure out is which one
can take a 2"+ mountain bike tire.

I wonder why the Sedona and Comfort 5 are rated more for comfort even
they have a smaller wheel diameter? Is it because of the wider tires
they come with or could it be because of the seat?

How far do I plan on riding? On a daily ride, they say you must
exercise your heart for 30 minutes. Including warm up and cool down,
that would probably be 35 minutes. How far can you go on a bike in 35
minutes? I really don't know.