My wife and I are in our 50's, and we are wondering what kind of physical conditioning we should do to prepare for taking lessons. We are weight in proportion to height, but are in need of shaping up I'm sure. What would you suggest.

Ok I'm only 26 but I just started dancing two months ago just for social purposes only (a new option to have fun and so I wont be the one person standing at the edge of a room at a function).
In my group lessons I dance with quite a few people your age and they are doing great. We all have pretty much the same problem with posture regardless of age.
Lessons move at a pace that allows your body to adjust so if you are only in it for the joy of dancing you should be fine without worrying too much about extra exercise.
I am relaxedly working out at a gym now: shoulders and back to stop my shoulders from slumping forward is what seems to help me the most.
A bad posture will tire you and your partner more than anything I have found.