I would only amend that to read:
I really can't recommend a perfect bike, because there isn't any such thing!

Every bike is designed for some purpose and it does that better than other things. Sure, you can try using your 29'r as a road bike and it will do it -- but not as well as the road bike. (and vice-versa).

Or, you could modernize and upgrade either the 29'r or the road bike with the latest performance components -- but I have the feeling that you still wouldn't have quite what you're thinking of...

So, why not consider something that could complement both the 29'r as well as the road bike? Perhaps a hybrid or tourer? Or, I am thinking that a Cyclo-cross might do that 70 miles really quickly...

Perhaps a Trek Cronus?
http://s7d4.scene7.com/is/image/Trek...0,0&iccEmbed=0