Mike - At 6 feet tall with a 32 inch inseam I would suspect you have a pretty long reach so keep that in mind when you are looking at bikes... I'm 5 foot ten (with a 32 inch inseam) and ride a 20 inch Trek 7500 Hybrid that I converted to a cross / tourer and love it.
A 20 inch bike might be perfect from a stand over point but you may find the cockpit to be a little cramped and I would really get rid of any flat bars as they are not well suited for distance riding as they really limit your hand positions.
40 miles is a fair ride for someone who is not used to it (even if they are in shape) as cycling uses different muscles... aerobically you may be fine but there are parts that might hurt a little afterwards.
Mostly... no matter what you buy... ride it like you stole it.