Exactly how "old and out of shape" do you consider yourself, brut4s? Most of the guys I ride with are approaching 40 with a few of them in their 50s and 60s. A dozen of us in town ride rigid single speeds, so one person's "old" isn't necessarily the next person's.
In the end, you're going to have to ride a bunch of different bikes and choose what's right for YOU, which you can do better than anybody. Most of us don't get it right the first time, either, so you can probably expect to buy a bike you believe will be perfect, only to sell it and buy another a year down the road.
I've known people to make the move like Mike did, from hardtail to FS and found it much more comfortable... but I've known just as many to go from years of FS riding to a hardtail and found THAT very comfortable and more fitting to their riding style. Me, for one. Flounder, same thing.
The one thing I will say... it's my opinion that anyone buying a new bike should get one with 29" wheels. For anyone who isn't (a) doing hardcore downhill or freeride on a 6" travel bike, or (b) isn't under 5'5", the advantages of 29" wheels are many, the disadvantages non-existent IMO.
Greg