Here's my two cents:
I went the other way. Started as a roadie, years on the rode bike a Trek, lots of miles by myself and lots of rides with fast groups. Then a few months ago I bought a Trek MTB with full suspension. I became convinced that I could ride it in the colder weather on trails than I might be able to ride the road bike out in the open. So, that I have done and I enjoy the MTB'ing immensely. Its a whole new set of skills I'm trying to develop, getting over logs, tree roots, short steep technical climbs.
But now, when I get back on my road bike it is so impressive, it feels like a Ferrari. So smooth, fast and controllable. A real joy to pedal. I love them both.
At the same time I wouldnt worry too much about pampering your body with friendly geometry, etc. I find you can do pretty much the same stuff you could years ago, you just have to slow it up a bit. I'm 56 by the way.
Cheers